Table 7

Montreal and Paris classification in Crohn’s disease

	Montreal29		Paris23
Age at diagnosis (years)	A1	<17	A1a	<10
	A1	<17	A1b	10–17
	A2	17–40	A2	17–40
	A3	>40	A3	>40
Location*	L1	Terminal ileal±limited caecal disease	L1	Distal 1/3 ileum±limited caecal disease
	L2	Colonic	L2	Colonic
	L3	Ileocolonic	L3	Ileocolonic
	L4	Isolated upper disease†	L4a	Upper disease proximal to ligament of Treitz†
	L4	Isolated upper disease†	L4b	Upper disease distal to ligament of Treitz and proximal to distal 1/3 ileum†
Behaviour	B1	Non-stricturing, non-penetrating	B1	Non-stricturing, non-penetrating
	B2	Stricturing	B2	Stricturing
	B3	Penetrating	B3	Penetrating
			B2B3	Both penetrating and stricturing disease, either at the same or different times
	P	Perianal disease modifier‡	P	Perianal disease modifier‡
Growth			G₀	No evidence of growth delay
Growth			G₁	Growth delay

B2: Stricturing defined as the occurrence of constant luminal narrowing demonstrated by radiologic, endoscopic, or surgical examination combined with pre-stenotic dilatation and/or obstructive signs or symptoms but without evidence of penetrating disease.
B3: Penetrating disease: defined as the occurrence of bowel perforation, intra-abdominal fistulae, inflammatory masses and/or abscesses at any time in the course of the disease, and not secondary to postoperative intra-abdominal complications (excludes isolated perianal or rectovaginal fistulae).
*Defined as endoscopic or macroscopic extent.
†Upper disease in Montreal classification describes disease proximal to the terminal ileum. In both Montreal and Paris: L4 and L4a/b may coexist with L1, L2, L3.
‡Perianal abscesses, ulcers or fistulae (but not skin tags or fissures).