We determined incidence of acute epiglottitis in the Northern California Region of the Kaiser Permanente Medical Care Program during an 11-year period, from 1980 through 1990. The health plan membership is representative of the ethnicity, age, sex, and occupation of the Northern California population. The study included 135 children and 258 adults. Annual incidence in adults was relatively stable, ranging from 1.16 to 2.12 cases per 100,000. Annual incidence in children, however, decreased from 3.47 cases per 100,000 in 1980 to 0.63 cases per 100,000 in 1990. The ratio of annual incidence in children to that in adults decreased from 2.6 in 1980 to 0.4 in 1990. This changing epidemiologic pattern may be the effect of the Hemophilus influenzae type b vaccine (Hib), which was first given to children in 1985. Acute epiglottitis, classically considered a disease of children, is now becoming a disease of adults.