Crohn’s disease hub
Where Crohn’s Happens: Locations & Behavior (Montreal Classification)
Last Updated Nov 11, 2025
Crohn’s disease is described by where it lives in the gut and how it behaves. The Montreal classification puts location into L1–L4 and behavior into B1–B3, with a “p” tag for perianal disease. Knowing someone’s L and B helps decide the best tests, set treatment targets, and flag risks like strictures or fistulas. It also allows teams to compare notes and track change over time. (academic.oup.com)
Key takeaways
Montreal uses L1–L4 for location and B1–B3 for behavior, plus “p” for perianal disease. (academic.oup.com)
Location tends to stay stable, behavior can progress from B1 to B2 or B3, especially with ileal involvement and smoking. (academic.oup.com)
Mild ileocecal (L1/right colon) flares may use controlled‑ileal‑release budesonide for induction, not for maintenance. (journals.lww.com)
Perianal “p” signals the need for pelvic MRI and combined medical‑surgical care, often with anti‑TNF therapy. (crohnscolitisfoundation.org)
Upper‑GI involvement (L4) needs upper endoscopy and small‑bowel imaging to map disease fully. (academic.oup.com)
Montreal at a glance
Montreal is a common language for Crohn’s teams. It records:
Age at diagnosis (A1–A3), used mainly for research and pediatrics.
Location (L1–L4).
Behavior (B1–B3), with “p” for perianal disease.
In adults, L4 is a modifier that can sit on top of L1–L3. Perianal disease is also a modifier, written as “p.” Example: A2 L3 B1p. (academic.oup.com)
Locations (L1–L4) and why they matter
L code | Where disease is | Common clues | Key tests | Treatment notes |
|---|---|---|---|---|
L1 | Terminal ileum, sometimes cecum | Cramping after meals, weight loss, anemia | Ileocolonoscopy with ileal intubation, MR enterography (MRE) or CT enterography to assess small bowel | Controlled‑ileal‑release budesonide can induce remission in mild ileocecal disease, but do not use for maintenance. Systemic therapies work throughout the gut when needed. (academic.oup.com) |
L2 | Colon only | Diarrhea, bleeding, urgency | Colonoscopy, plus imaging if symptoms suggest upstream disease | Use systemic therapies when disease is moderate to severe. Sulfasalazine may be considered only for symptomatic mild colonic Crohn’s. (guidelinecentral.com) |
L3 | Ileocolonic | Mix of L1 and L2 features | Combo of colonoscopy and small‑bowel imaging | Often needs systemic therapy, guided by targets and response. (academic.oup.com) |
L4 | Upper GI (esophagus, stomach, duodenum, proximal jejunum/ileum) | Nausea, vomiting, early fullness, upper‑abdomen pain | Upper endoscopy, small‑bowel imaging; capsule endoscopy if appropriate | L4 is a modifier that changes testing plans. Medical therapy is systemic; imaging helps detect strictures. (academic.oup.com) |
Imaging choices depend on availability and symptoms. MRE and intestinal ultrasound avoid radiation and are preferred for repeated assessments. Capsule endoscopy can see proximal small bowel if strictures are excluded. (academic.oup.com)
Behaviors (B1–B3) and the “p” modifier
B1, non‑stricturing, non‑penetrating: Active inflammation without scarring or tunnels. Symptoms may wax and wane. Aim for clinical and endoscopic remission under treat‑to‑target. (pubmed.ncbi.nlm.nih.gov)
B2, stricturing: Narrowed segments cause cramping, bloating, and sometimes vomiting. Cross‑sectional imaging helps define length and severity. Medicines treat inflammatory swelling; fixed scar tissue may need endoscopic dilation or surgery. No test reliably separates pure scar from inflammation every time. (academic.oup.com)
B3, penetrating: Fistulas or abscesses can connect bowel to bowel, bladder, skin, or other spaces. Care often combines antibiotics, drainage of abscesses, and advanced medicines, with surgery when needed. (academic.oup.com)
“p,” perianal modifier: Signals fistulas or abscesses near the anus. Best practice includes pelvic MRI and exam under anesthesia when needed, drainage of any abscess, and usually anti‑TNF therapy, often with setons placed by a colorectal surgeon. Chronic seton alone is not preferred as the only therapy. (crohnscolitisfoundation.org)
How location and behavior guide care
Tests: L1–L3 usually start with ileocolonoscopy, plus MRE or CT enterography to see the small bowel. L4 adds upper endoscopy. Perianal “p” adds pelvic MRI or endoanal ultrasound. (academic.oup.com)
Medicines: Most modern therapies act throughout the gut. Location matters for targeted delivery, for example controlled‑ileal‑release budesonide for mild ileocecal disease. Behavior matters for urgency and mix of medical and surgical care. (journals.lww.com)
Treat‑to‑target: Teams track symptoms, biomarkers, and endoscopic healing, adjusting therapy if targets are not met. This approach aims to prevent complications like strictures and fistulas. (pubmed.ncbi.nlm.nih.gov)
Does behavior change over time
Yes. Location usually stays the same, while behavior can shift from B1 to B2 or B3. In population‑based cohorts, about half of people develop a stricturing or penetrating complication over long follow‑up. Risks include ileal involvement and smoking. Early objective control reduces downstream damage. (academic.oup.com)
Some newer studies describe a “rolling phenotype,” noting that with sustained control, people can spend long periods free of complications. The Montreal system still records the most severe behavior seen at any time. (pubmed.ncbi.nlm.nih.gov)
Recording Montreal in real life
Clinicians document A, L, and B at diagnosis, then update behavior if complications occur. Examples:
A2 L1 B1
A2 L3+L4 B2
A1 L2 B1p
Pediatrics often uses the Paris system, which further splits upper‑GI disease and age groups. (journals.lww.com)
FAQs
Why does “p” perianal disease change the plan
Perianal disease needs imaging of the pelvis, prompt drainage of any abscess, and combined medical‑surgical care. Anti‑TNF therapy has the strongest evidence for fistula healing, often alongside seton placement. (journals.lww.com)
Can budesonide keep mild ileal Crohn’s in remission
No. It can induce remission in mild ileocecal disease, but it is not effective for long‑term maintenance. A steroid‑sparing plan is needed. (journals.lww.com)
How do teams decide when to escalate therapy
They use treat‑to‑target. If symptoms, biomarkers, or endoscopic findings do not reach agreed targets, therapy is adjusted to prevent complications. (pubmed.ncbi.nlm.nih.gov)