To describe the epidemiology of appendicitis and appendectomy in the United States, the authors analyzed National Hospital Discharge Survey data for the years 1979-1984. Approximately 250,000 cases of appendicitis occurred annually in the United States during this period, accounting for an estimated 1 million hospital days per year. The highest incidence of primary positive appendectomy (appendicitis) was found in persons aged 10-19 years (23.3 per 10,000 population per year); males had higher rates of appendicitis than females for all age groups (overall rate ratio, 1.4:1). Racial, geographic, and seasonal differences were also noted. Appendicitis rates were 1.5 times higher for whites than for nonwhites, highest (15.4 per 10,000 population per year) in the west north central region, and 11.3% higher in the summer than in the winter months. The highest rate of incidental appendectomy was found in women aged 35-44 years (43.8 per 10,000 population per year), 12.1 times higher than the rate for men of the same age. Between 1970 and 1984, the incidence of appendicitis decreased by 14.6%; reasons for this decline are unknown. A life table model suggests that the lifetime risk of appendicitis is 8.6% for males and 6.7% for females; the lifetime risk of appendectomy is 12.0% for males and 23.1% for females. Overall, an estimated 36 incidental procedures are performed to prevent one case of appendicitis; for the elderly, the preventive value of an incidental procedure is considerably lower.