The epidemiology of Crohn's disease (CD) was investigated among ethnic groups in a 2-million-member health maintenance organization. Between 1982 and 1988 there were 909 hospitalizations for CD. Annual hospitalizations per 100,000 were much lower for Hispanics (0.6) than those for whites (10.2) and blacks (10.2), and rates were higher among women (8.3) than men (6.0) (P less than 0.001). Bimodal age distributions were found for both sexes with peaks identified in the 20-29-year and greater than or equal to 60-year age groups. Annual age-adjusted hospitalizations per 100,000 decreased from 1982 (8.3) through 1988 (5.4) (P less than 0.05). A mail survey and medical records were used to collect data from the 169 CD patients who were identified at two sites within the organization. Prevalence rates per 100,000 for Hispanics (4.1) and Asians (5.6) were much lower than those for whites (43.6), blacks (29.8), and "others" (8.4). The distribution of age at diagnosis was bimodal, and the average age at diagnosis was 36 years. Patients reported an average of 3.7 outpatient visits and 13.3 days lost from work per year. Current cigarette smokers reported significantly more days troubled by symptoms during a one-month period (15.4 days) than nonsmokers (5.0 days) (P less than 0.001).