Definition of Crohn's disease phenotype according to the Montreal classification23
| Age of onset | Location | Behaviour |
| ≤16 years (A1) | Ileal (L1) | Non-stricturing, |
| Non-penetrating (B1) | ||
| 17−40 years (A2) | Colonic (L2) | Stricturing (B2) |
| >40 years (A3) | Ileo-colonic (L3) | Penetrating (B3) |
| *Isolated upper GI disease (L4) | + ‘p’ if peri-anal disease |
↵* L4 is a modifier that can be added to L1 – 3 when concomitant upper gastrointestinal (GI) disease is present.