The incidence of acute appendicitis has been reported to vary substantially by country, geographic region, race, sex, season, and occupation, but the reasons for this variation are unknown. We evaluated several risk factors for appendicitis by analyzing data from hospital discharge abstracts on all cases of acute appendicitis treated surgically in nonfederal hospitals in California from 1983 to 1986 (N = 102,546). Comparison of age and sex-specific incidence rates for four racial/ethnic groups (white, Hispanic, black, and Asian/other) revealed rates in blacks and the Asian/other group one-half or less of the rates for whites and Hispanics between the ages of 5 and 29 years. Incidence rates in males were higher than rates in females in all racial/ethnic groups for most ages (RR = 1.1-1.7). Seasonal variation in incidence was modest. Peak rates occurred in July, August, and September, and the lowest rates occurred in December. Hypotheses about the etiology of appendicitis must account for substantial racial/ethnic, gender, age variation, and modest seasonal variation in the incidence.