['The Montreal classification system of inflammatory bowel disease (IBD) provides a framework for describing disease phenotype.', 'We aimed to describe changes in IBD phenotype using the Montreal system and determine predictors of phenotype change in a Caucasian population-based cohort.', 'Ninety-two percent of people with IBD in Canterbury, New Zealand were recruited. Clinical notes were reviewed to confirm diagnosis and phenotype. Determinants of phenotype change were analyzed using multivariate analysis.', "A total of 1,420 (715 Crohn's disease [CD], 668 ulcerative colitis [UC]) patients with IBD were included. Median follow-up was 6.5 and 10.9 yr for CD and UC, respectively. Disease location remained stable in 91% of those with CD. Seventy-three percent of CD patients had inflammatory disease at diagnosis with the proportion of patients with complicated disease increasing over time. Progression to complicated disease was more rapid in those with small bowel than colonic disease location, (P < 0.001). Perianal disease was a significant predictor of change in CD behavior (HR 1.62, P < 0.001). Younger UC patients were more likely to have extensive disease at diagnosis than older patients (P < 0.001)."] Does perianal disease predict changes in Crohn 's disease phenotype-results of a population-based study of inflammatory bowel disease phenotype?