Introduction
When you're told you have Crohn's disease, that diagnosis alone doesn't tell the full story. Crohn's is classified along two separate axes: where the disease is located in your gastrointestinal tract, and how it behaves. These two dimensions combine to form your specific disease phenotype, and understanding yours matters because it directly shapes which treatments your gastroenterologist will recommend and what your long-term outlook looks like.
How Crohn's Disease Is Classified
Gastroenterologists use the Montreal classification system to categorize Crohn's disease. This framework evaluates three variables: age at diagnosis, disease location, and disease behavior. Age at diagnosis is divided into three groups: 16 years or younger, 17 to 40, and over 40. But the two variables that have the most practical impact on your care are location and behavior.
Location refers to the segment of the GI tract affected. Behavior describes what the inflammation is doing to the tissue. Your disease is classified by both, and the combination creates a more complete picture than either variable alone.
The Five Location Types
Crohn's disease can affect any part of the digestive tract from mouth to anus, but it tends to cluster in specific areas. The five recognized types by location are:
Ileocolitis affects the end of the small intestine (the ileum) and part of the large intestine. It is the most common form, accounting for roughly half of all Crohn's diagnoses.
Ileitis involves only the ileum. Symptoms overlap significantly with ileocolitis, including pain in the lower right abdomen, diarrhea, and weight loss.
Crohn's colitis (also called granulomatous colitis) affects only the colon. It can resemble ulcerative colitis, which sometimes makes initial diagnosis tricky.
Two less common types round out the list. Gastroduodenal Crohn's disease involves the stomach and the first part of the small intestine (the duodenum), causing nausea, vomiting, and loss of appetite. Jejunoileitis affects the upper half of the small intestine (the jejunum) and can cause patchy areas of inflammation separated by healthy tissue.
The Montreal system also recognizes upper GI involvement as a modifier, meaning a patient with ileocolitis who also has gastroduodenal disease would receive both designations. Disease location tends to remain relatively stable over time, though it can extend in some patients.
The Three Behavior Types
While location tells you where Crohn's disease is, behavior tells you what it's doing. This distinction drives treatment decisions more than location alone. The three behavior categories are inflammatory, stricturing, and penetrating.
Inflammatory behavior (designated B1 in the Montreal system) means the disease causes inflammation without structural complications. This is the most common presentation at diagnosis and typically responds best to medical therapy, including biologics and immunomodulators.
Stricturing behavior (B2) means chronic inflammation has caused the bowel wall to thicken and narrow, creating strictures that can partially or fully block the intestine. Patients with stricturing disease may experience cramping after meals, bloating, and changes in stool caliber. Treatment often requires endoscopic dilation or surgical resection in addition to medication.
Penetrating behavior (B3) is the most aggressive phenotype. The inflammation has burrowed through the full thickness of the bowel wall, forming abnormal connections called fistulae between the intestine and other structures, such as the skin, bladder, or other bowel loops. Abscesses can also develop. Penetrating disease typically requires aggressive medical therapy combined with surgical intervention.
Why Crohn's Disease Doesn't Have "Stages"
Many patients search for "stages of Crohn's disease," expecting a linear progression similar to cancer staging. Crohn's doesn't work that way. Cancer stages represent a fixed progression: stage I is localized, stage IV has spread. Crohn's disease severity fluctuates over time, cycling between flares and periods of remission. A patient classified as "moderate" today may be in remission next month and flaring again six months later.
What does progress in a more directional way is disease behavior. Research shows that over 50% of patients who start with purely inflammatory disease will develop stricturing or penetrating complications during long-term follow-up. Approximately 70% of Crohn's patients will develop fibrotic strictures within 10 years of diagnosis. Risk factors for this progression include ileal disease location, smoking, and frequent flares.
This behavioral evolution is one of the strongest arguments for early, effective treatment. Controlling inflammation before it causes structural damage to the bowel wall can help prevent the transition from inflammatory to stricturing or penetrating disease.
What This Means for Your Care
Knowing your specific disease classification, both location and behavior, gives you a clearer picture of what to expect and what questions to ask your gastroenterologist. A patient with ileal stricturing disease faces different treatment decisions than someone with colonic inflammatory disease.
Ask your care team which Montreal classification applies to you. Track the symptoms that correspond to your disease type, whether that's obstructive symptoms for stricturing disease, drainage or fever for penetrating disease, or frequency and urgency for colonic involvement. The more precisely you can describe what's happening, the more effectively your team can adjust your treatment plan.