BACKGROUND: There are few contemporary estimates of prevalence rates for inflammatory bowel disease (IBD) in diverse North American communities. METHODS: We estimated the period prevalence of IBD for January 1, 1999, through June 30, 2001, among 1.8 million randomly sampled members of nine integrated healthcare organizations in the US using computerized diagnoses and outpatient pharmaceutical dispensing. We also assessed the positive predictive value (PPV) and sensitivities of 1) the case-finding algorithm, and 2) the 30-month sampling period using medical chart review and linkage to a 78-month dataset, respectively. RESULTS: The PPV of the case-finding algorithm was 81% (95% confidence interval [CI], 78-87) and 84% (95% CI, 79-89) in two different organizations. In both, the sensitivity of the optimal algorithm, compared with the most inclusive, exceeded 90%. The sensitivity of the 30-month sampling period compared with 78 months was 61% (95% CI, 57-64) in one organization. Applying a slightly more sensitive case-finding algorithm, the average period prevalence of IBD across the nine organizations, standardized to the age- and gender-distribution of the US population, 2000 census, was 388 cases (95% CI, 378-397) per 100,000 persons (range 209-784 per 100,000; average follow-up 26 months). The prevalence of Crohn’s disease, ulcerative colitis, and unspecified IBD was 129, 191, and 69 per 100,000, respectively. CONCLUSIONS: The observed average prevalence was similar to prevalence proportions reported for other North American populations (369-408 per 100,000). Additional research is needed to understand differences in the occurrence of IBD among diverse populations as well as practice variation in diagnosis and treatment of IBD.