Where Crohn’s Happens: Locations & Behavior (Montreal Classification)

Crohn’s disease affects different parts of the digestive tract in different ways. The Montreal Classification gives a simple code for where disease is located (L1–L4) and how it behaves (B1–B3, with a perianal modifier). These patterns help care teams estimate long‑term risk and choose treatments, from medication intensity to surgery, nutrition planning, and cancer surveillance.

Key Takeaways

• The Montreal Classification is a shorthand that describes age at diagnosis, location, and behavior of Crohn’s disease.

• L1–L4 describe where Crohn’s inflammation lives, from the end of the small bowel to the colon and upper digestive tract.

• B1–B3 describe how Crohn’s behaves over time: purely inflammatory, stricturing (narrowing), or penetrating (fistulas and abscesses).

• A small “p” modifier is added when there is perianal Crohn’s, which often signals a more complicated disease course.

• High‑risk patterns like L3, L4, B2/B3, and perianal disease often lead to earlier advanced therapy and closer monitoring.

• Montreal codes guide treatment planning, but they do not replace ongoing assessment of symptoms, labs, imaging, and personal goals.

What the Montreal Classification Is

The Montreal Classification is an international system used to describe Crohn’s disease in a standard way. It has three main parts:

• A for age at diagnosis

• L for location in the digestive tract

• B for behavior (how the disease damages the bowel over time), plus a “p” modifier for perianal disease (sapd.es)

This system is used in clinic notes and research. It helps compare groups of people with Crohn’s, estimate risks, and shape treatment strategies.

Over time, the location of Crohn’s usually stays fairly stable, while behavior often changes from purely inflammatory to stricturing or penetrating in a significant share of people. (journals.lww.com)

Location: L1–L4

Location codes describe which part of the gut is inflamed. A person may have more than one affected area.

Definitions follow the original Montreal and pediatric Paris descriptions. (abdominalkey.com)

L1: Terminal ileum

L1 means Crohn’s mainly affects the terminal ileum, the last stretch of small intestine, sometimes with limited cecal involvement. (abdominalkey.com)

Common features:

• Cramping or pain in the lower right abdomen

• Diarrhea and weight loss

• Risk of vitamin B12 deficiency and bile acid–related diarrhea after repeated inflammation or surgery in this area

People with ileal or ileocolonic disease are more likely to develop strictures or penetrating complications compared with those who have isolated colonic disease. (journals.lww.com)

L2: Colon

L2 describes Crohn’s limited to the colon. (sapd.es)

Typical features:

• Diarrhea, often with blood or mucus

• Urgency and crampy lower abdominal pain

• Over many years, increased risk of colorectal cancer when colonic inflammation is chronic and extensive (journals.lww.com)

Because colonic Crohn’s can resemble ulcerative colitis, endoscopy and biopsies are important to confirm the pattern.

L3: Ileocolon

L3 means inflammation in both the terminal ileum and the colon. (sapd.es)

This is one of the most common patterns and often carries a higher risk of:

• Intestinal complications (strictures, fistulas)

• Abdominal surgery

• Surgical recurrence when combined with penetrating and perianal disease (for example, L3 B3p) (pubmed.ncbi.nlm.nih.gov)

Because both small and large bowel are involved, care teams watch for malnutrition, vitamin deficiencies, and colon‑cancer risk.

L4: Upper gastrointestinal tract

L4 refers to Crohn’s disease above the terminal ileum, such as the esophagus, stomach, duodenum, jejunum, or proximal ileum. (pubmed.ncbi.nlm.nih.gov)

Key points:

• L4 can be isolated (L4 alone) or added on to L1, L2, or L3 (for example, L3 + L4). (radiopaedia.org)

• Jejunal involvement within L4 is linked to a higher risk of surgery and a more challenging course. (pubmed.ncbi.nlm.nih.gov)

• There is a higher concern for malabsorption, weight loss, and micronutrient deficiencies in people with more proximal small bowel disease.

Across all locations, most people keep the same L code over time, with only a small minority showing true extension to new regions. (journals.lww.com)

Behavior: B1–B3 and the Perianal “p”

Behavior codes describe how Crohn’s damages the bowel:

These definitions come from the Montreal and Paris classifications and are widely used in guidelines. (sapd.es)

B1: Inflammatory (non‑stricturing, non‑penetrating)

B1 is active inflammation without permanent narrowing or abnormal passages.

Features:

• Pain, diarrhea, and bleeding driven by inflammation

• Often responds well to medical therapy when treated early

• Over years, a notable share of B1 disease progresses to stricturing or penetrating behavior (journals.lww.com)

B2: Stricturing

B2 describes fixed narrowing of the bowel from scarring. It is defined on imaging or endoscopy as persistent luminal narrowing with pre‑stenotic dilation or symptoms of obstruction, without features of penetrating disease. (abdominalkey.com)

Common issues:

• Crampy post‑meal pain, bloating, and sometimes vomiting

• Partial or complete bowel obstruction

• Need to decide whether a stricture is mainly inflammatory (may improve with medical therapy) or fibrotic (often needs endoscopic dilation or surgery)

B3: Penetrating

B3 refers to disease that breaks through the bowel wall, leading to:

• Fistulas between loops of bowel or from bowel to other organs or skin

• Intra‑abdominal abscesses or phlegmons

• Spontaneous perforation not related to surgery (abdominalkey.com)

Penetrating behavior is associated with more hospitalizations and surgeries than purely inflammatory disease. (pubmed.ncbi.nlm.nih.gov)

Perianal modifier “p”

Perianal Crohn’s includes fistulas, abscesses, or complex skin changes around the anus. In the Montreal system, this is written as a small “p” added to B1, B2, or B3, such as B2p or B3p. (pmc.ncbi.nlm.nih.gov)

Perianal disease:

• Affects up to about one quarter of people with Crohn’s over time (journals.lww.com)

• Is a marker of more aggressive disease and higher surgical risk, especially when combined with ileocolonic and penetrating disease (for example, L3 B3p). (pubmed.ncbi.nlm.nih.gov)

How Location and Behavior Guide Treatment

Montreal codes are one piece of risk stratification. Guidelines highlight several features linked with a higher risk of progressive Crohn’s:

• Ileal, ileocolonic, or upper GI (L1, L3, L4) involvement

• Extensive bowel involvement and deep ulcers

• Perianal or severe rectal disease (p)

• Stricturing or penetrating behavior at diagnosis (B2 or B3)

• Younger age at diagnosis and certain extraintestinal manifestations (journals.lww.com)

These patterns often lead care teams to:

• Use earlier advanced therapies (such as biologics or small‑molecule drugs) instead of waiting for multiple flares or surgeries.

• Monitor more closely with imaging, endoscopy, and biomarkers to detect complications early.

Examples of how Montreal patterns shape care:

• L1 or L3 with B2 (stricturing)

• Extra attention to obstruction symptoms.

• Imaging to characterize strictures, with possible balloon dilation or surgery for fixed fibrotic narrowing. (abdominalkey.com)

• L3 or L4 with B3 (penetrating) and/or p

• Often managed with both surgical or interventional drainage and targeted medications such as anti‑TNF therapy or other biologics.

• Pelvic MRI and colorectal surgery input are common when perianal disease is present. (journals.lww.com)

• L2 or L3 with long‑standing colonic inflammation

• Regular colonoscopy surveillance for colorectal cancer is recommended after several years of colitis, taking into account extent, duration, and coexisting conditions like primary sclerosing cholangitis. (journals.lww.com)

• L4 or extensive small bowel involvement

• Early involvement of nutrition specialists to monitor weight, vitamin status, and bone health.

• Increased awareness of the small but elevated risk of small bowel adenocarcinoma in long‑standing small bowel Crohn’s. (journals.lww.com)

Even with the same Montreal code, treatment is individualized based on symptoms, response to prior therapies, other medical conditions, and personal preferences.

What the Montreal Classification Cannot Do

The Montreal system has limits:

• It does not grade current disease activity or symptom burden.

• It does not capture depth of ulcers, severity scores, or extraintestinal manifestations such as joint, skin, or eye involvement.

• Behavior can evolve over time. Many people move from B1 to B2 or B3, and complications can sometimes improve or resolve with treatment or surgery. (journals.lww.com)

Researchers have even proposed “rolling” phenotypes that track behavior over recent years rather than a single label, to reflect this changing course. (pubmed.ncbi.nlm.nih.gov)

Because of these limits, Montreal codes are best seen as a framework, not a prediction. Ongoing follow‑up, updated imaging, and shared decision making remain central to care.

FAQs

Does the Montreal Classification ever change over time?

Yes. Location (L) tends to be stable, but behavior (B) changes in many people. Most are B1 at diagnosis, but up to about half develop stricturing or penetrating complications within 20 years. When that happens, the B code is usually updated in the medical record. (journals.lww.com)

Is one Montreal pattern “worse” than another?

Patterns with stricturing or penetrating behavior (B2/B3), perianal disease (p), and more extensive locations (L3/L4) are linked with more complications, surgeries, and hospitalizations compared with limited inflammatory disease. (journals.lww.com)
However, timely use of effective therapy can improve outcomes across all patterns.

How is a person’s Montreal Classification determined?

The classification is based on:

• Endoscopy (colonoscopy, sometimes upper endoscopy)

• Cross‑sectional imaging like MR enterography or CT enterography

• Surgical findings, when relevant

Care teams combine these results to assign L and B categories and the perianal modifier. Different clinicians sometimes disagree on details, which is why clear imaging and endoscopy reports are important. (pmc.ncbi.nlm.nih.gov)

Can treatment change behavior from B2 or B3 back to B1?

Medical and surgical treatments can heal fistulas, drain abscesses, and open inflamed strictures, so the day‑to‑day picture may look more like B1 again. However, permanent scarring or prior complications still matter, and many studies classify behavior based on the worst complication that has occurred, not just the current state. (pubmed.ncbi.nlm.nih.gov)