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The epidemiology of inflammatory bowel disease is described among the more than 1.5 million members of the Kaiser Permanente Medical Care Program (KPMCP) in northern California. We reviewed a 20% random sample of the medical records of 2,067 persons first admitted to hospital in the period 1971 through 1982 with codes indicating inflammatory bowel disease. We also examined all new outpatient cases for a 1-year period from records at the Oakland KPMCP facility. Criteria used to establish valid cases adhered to standards used in previous studies but were revised to reflect current diagnostic methods. The disadvantages of using hospital discharge data have been identified and quantified. In this population, only 21% of ambulatory patients with inflammatory bowel disease were admitted to hospital in a 3 1/2-year period. There was no difference in the incidence of the disorder by sex or between whites and blacks, but it was rare in Asians. A bimodal age distribution was suggested for Crohn's disease but not for ulcerative colitis. During the 12 years of this study, rates of hospital admissions for ulcerative colitis decreased and for Crohn's disease were slightly increased.