Dr. Elizabeth Squirell

Montreal classification

Montreal phenotype builder for Crohn's disease and UC, with the Paris paediatric version

Reviewed October 2026

For healthcare professionals. A decision aid, not a substitute for clinical judgement.

Code the age at diagnosis, location and behaviour, with p for perianal disease.

Version

ClassificationFor paediatric-onset disease, switch to Paris.

Age at diagnosis

Age at diagnosisA1 16 or younger · A2 17–40 · A3 over 40 (completed years).
years
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Location

Macroscopic involvement on endoscopy and cross-sectional imaging, not histology alone.

Ileum and colon
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Upper GI disease?Disease proximal to the terminal ileum. L4 is added to L1–L3, or stands alone.
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Behaviour

Ever present, at any time since diagnosis.

Stricturing disease?Constant luminal narrowing on imaging, endoscopy or surgery, with prestenotic dilatation or obstructive symptoms.
Penetrating disease?Bowel perforation, intra-abdominal fistula, inflammatory mass or abscess. Not postoperative complications, and not isolated perianal or rectovaginal fistulas.
Perianal disease? (p modifier)Perianal fistula, abscess or anal canal ulcer. Skin tags alone do not count.
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Montreal classification

Answer 6 more items to see the result.

  • Age at diagnosis
  • Ileocolonic location
  • Upper GI disease (answer yes or no)
  • Stricturing disease (answer yes or no)
  • Penetrating disease (answer yes or no)
  • Perianal disease (answer yes or no)

How to use it

Pick the disease, then answer plain questions. The tool builds the code. For example, A2 L3+L4 B2p means diagnosed at 17–40, ileocolonic with upper GI disease, stricturing, with perianal disease. Behaviour is asked as two yes/no questions (stricturing ever? penetrating ever?), so the most-advanced rule is applied for you.

Use Montreal for adults. Switch to Paris for paediatric-onset disease. Paris keeps the Montreal framework, so an adult-onset patient gets almost the same code.

The codes

Crohn’s disease Montreal Paris
Age at diagnosis A1 16 or younger · A2 17–40 · A3 over 40 A1a under 10 · A1b 10 to under 17 · A2 17–40 · A3 over 40
Location L1 terminal ileum · L2 colon · L3 ileocolon · L4 upper GI, added to L1–L3 or alone L1 distal third of ileum ± limited caecal disease · L2 · L3 · L4a proximal to the ligament of Treitz · L4b distal to it, above the distal ileum
Behaviour B1 non-stricturing, non-penetrating · B2 stricturing · B3 penetrating as Montreal, plus B2B3 for both
Modifiers p perianal disease p perianal disease · G0/G1 growth
Ulcerative colitis Montreal Paris
Extent E1 proctitis · E2 left-sided, distal to the splenic flexure · E3 extensive, proximal to the splenic flexure E1 · E2 · E3 extensive to the hepatic flexure · E4 pancolitis, proximal to the hepatic flexure
Severity S0 remission · S1 mild · S2 moderate · S3 severe S0 never severe · S1 ever severe (PUCAI 65 or more)

Montreal S3 is at least 6 bloody stools a day with a pulse of 90 or more, temperature 37.5 °C or more, hemoglobin below 105 g/L and ESR 30 mm/h or more. S1 is 4 or fewer stools a day with no systemic illness and a normal ESR. S2 lies between.

Behaviour: the most advanced phenotype, reassessed over time

  • Assign the most advanced behaviour ever seen. Montreal codes a patient with both strictures and penetrating disease as B3. Paris records both as B2B3, because the two can arise separately.
  • Behaviour changes; location mostly doesn’t (ACG 2018). About 20% of adults have stricturing or penetrating disease at diagnosis, but after 20 years only 12% still have a purely inflammatory phenotype (data cited by Levine 2011). B1 is often a temporary label. Montreal leaves the time point for classification to each study (Satsangi 2006). In practice, reclassify at each major reassessment and record the disease duration with the code.
  • Location can extend in children. In one paediatric cohort, 39% of those without maximal extent had new sites within 2 years (Levine 2011). Paris recommends regular updates of location and behaviour.

Perianal disease is a modifier

The p sits beside any behaviour (B1p, B2p, B3p). Perianal fistulising disease doesn’t necessarily come with intestinal fistulising disease, so it isn’t B3 by itself (Satsangi 2006). Paris excludes isolated perianal or rectovaginal fistulas from B3. It uses p only for a fistula, abscess or anal canal ulcer, not skin tags (Levine 2011). The code doesn’t grade perianal activity: use the PDAI.

Pitfalls

  • Code from macroscopic disease on endoscopy and imaging, not from histology alone in normal-looking mucosa (Paris). You can only code upper GI or small-bowel disease in segments that were actually examined.
  • L4 isn’t a separate location in Montreal. It is added to L1–L3 (for example L3+L4), and stands alone only for isolated upper disease.
  • UC severity in Montreal describes one point in time. Paris S1 means ever severe, so it never goes back to S0. For acute severe colitis now, use the ASUC tool.
  • The classification describes phenotype, not activity. Pair it with an activity score: CDAI and PRO2 or the HBI, and SES-CD for endoscopy.

Try “pregnancy”, “Mayo”, “calprotectin” or “VARSITY”.