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International cohort studies have reported increased incidence of inflammatory bowel disease (IBD) in recent years, and Canada has among the highest rates of IBD in the world. This study assessed incidence and prevalence of IBD in Ontario, the most populous province of Canada, to determine changing trends in age of onset.We used a population-based cohort derived from validated health administrative data consisting of all Ontario residents living with IBD from 1999 to 2008. We determined trends over time using Poisson regression analysis, assessing rates in 10-year age groups, children, adults, and the elderly.In 2008, 68,071 people were living with IBD among 12,738,350 people (standardized prevalence 534.3 per 100,000 people). Between 1999 and 2008, standardized IBD incidence increased from 21.3 to 26.2 per 100,000 (2.3% per yr, P < 0.0001). Incidence of Crohn's increased from 9.6 to 12.1 per 100,000 (1.9% per yr, P < 0.0001). Ulcerative colitis incidence increased from 10.7 to 12.1 per 100,000 (2.0% per yr, P < 0.0001). For IBD, incidence increased significantly in people younger than 10 years of age (9.7% per yr, P < 0.0001), 10 to 19 years of age (3.8% per yr, P < 0.0001), 30 to 39 years of age (1.8% per yr, P = 0.0006), 40 to 49 years of age (2.8% per yr, P = 0.0001), and 50 to 59 years of age (2.8% per yr, P < 0.0001). Incidence was stable in patients older than 65 years of age at diagnosis (−0.1% per yr, P = 0.73). Although incidence did not change significantly over time in adults 20 to 29 years, IBD incidence peaked in this age group.Ontario has among the highest prevalence of IBD in the world. Incidence of IBD increased between 1999 and 2008, owing to increased incidence in children and adults, with stable rates in elderly people. These findings demonstrate the changing age demographics and growing burden of IBD in Ontario, Canada.