Terminal ileum and any location between caecum and rectum.
B1
Non-stricturing, Non-penetrating
Inflammatory pattern only.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Phenotypic characterization of Crohn's disease for research and registry purposes
To predict the long-term clinical course and risk of surgery
Standardizing communication between IBD specialists regarding a patient's "ALB" phenotype (Age, Location, Behavior)
Philosophy of Montreal
The Montreal Classification (2005) is an update of the Vienna Classification (1998). It recognizes that Crohn's is a heterogeneous disease where the "age at diagnosis" and "behavior" (stricturing/penetrating) significantly influence prognosis.
Section 2
Formula & Logic
A: Age at Diagnosis
A1
≤ 16 years (Pediatric-onset)
A2
17–40 years
A3
> 40 years
L: Disease Location
L1
Ileal (Limited to the terminal ileum / caecum)
L2
Colonic (No ileal or upper GI involvement)
L3
Ileocolonic (Involvement of both)
L4
Upper GI modifier (Add to L1-L3)
B: Disease Behavior
B1
Non-stricturing, non-penetrating (Inflammatory)
B2
Stricturing
B3
Penetrating (Fistulas or abscesses)
p Modifier
Add if perianal disease is present (e.g., B2p)
Section 3
Pearls/Pitfalls
The Significance of "A1"
Pediatric-onset Crohn's (A1) is frequently associated with a more aggressive disease course, a higher rate of ileocolonic involvement (L3), and a stronger genetic predisposition than adult-onset disease.
Behavioral Progression
Crohn's disease often progresses from B1 (inflammatory) to B2 or B3 over time. A patient's Montreal stage should be updated throughout their life; it is a "living" phenotypic record.
Clinical Pearls
L1 (Ileal) disease is the most common phenotype to present with B2 (Stricturing) complications
Perianal disease (p) is now considered a "modifier" rather than a primary behavior, acknowledging it co-exists with L1-L3
The Upper GI (L4) modifier is added if any disease is found proximal to the terminal ileum (e.g., esophagus, stomach, or proximal small bowel)
Section 4
Next Steps
Prognostic Integration
01
B2/B3 Phenotype: High risk of surgery. Favor "Top-down" biological therapy.
02
A1 or L3 Phenotype: Correlates with more extensive disease burden.
Complementary Scoring
Montreal Classification (UC)
SES-CD (Endoscopic Activity)
Lémann Index (Cumulative Damage)
Section 5
Evidence Appraisal
The Definitive Paper
Toward an integrated clinical, molecular and serological classification of inflammatory bowel disease: Report of a Working Party of the 2005 Montreal World Congress of Gastroenterology.
Silverberg MS et al. • Canadian Journal of Gastroenterology. 2005;19 Suppl A:5A-36A. The primary source for the A-L-B criteria.
Finalized at the World Congress of Gastroenterology in Montreal (2005), this system harmonized definitions that had been fragmented for decades. It remains the universal standard for reporting phenotype in IBD clinical trials.
