OBJECTIVE: There are few estimates of the incidence and prevalence of inflammatory bowel disease in North American communities. We sought to estimate the incidence and prevalence of inflammatory bowel disease (IBD), including Crohn’s disease (CD), and ulcerative colitis (UC), among 3.2 million members of Kaiser Permanente, Northern California, for the period 1996-2002. METHODS: All health plan members who had one or more diagnoses of CD (ICD-9 code 555) or UC (ICD-9 code 556) on computerized records during the period 1996-2002 and with at least 12 months of membership were identified as possible IBD cases (N = 12,059). We randomly sampled 24% of these for chart review to confirm the diagnosis and obtain the initial diagnosis date. Incidence rates and the point prevalence on December 31, 2002 were standardized to the 2000 U. S. Census. RESULTS: The annual incidence rate per 100,000 persons was 6.3 for CD (95% confidence interval [CI], 5.6-7.0) and 12.0 for UC (CI, 11.0-13.0). The point prevalence per 100,000 on December 31, 2002 was 96.3 for CD (95% CI, 89.6-103.0) and 155.8 for UC (95% CI, 146.6-164.9), increasing to 100.3 and 205.8 per 100,000, respectively, when hospital discharge data from 1985 to 1995 were included. The age-specific incidence of CD was bimodal, while UC incidence rose in early adulthood and remained elevated with advancing age. CONCLUSIONS: The incidence we estimated for CD was similar to the previous U. S. estimate. Our incidence estimate for UC was much higher than the previous U.S. estimate, but similar to that of recent Canadian and European studies. The prevalence we estimated for CD was somewhat lower than previous estimates.