To determine trends in the incidence and epidemiology of acute hepatitis B in the United States we conducted intensive surveillance for viral hepatitis in four sentinel counties from October 1, 1981, to September 30, 1988. The overall incidence of hepatitis B remained relatively constant throughout the study period (average, 13.2 cases per 100,000 population), but disease transmission patterns changed significantly. The proportions of hepatitis B cases accounted for by homosexual activity and health care employment decreased 62% and 75%, respectively; the proportions of cases accounted for by parenteral drug use and heterosexual exposure increased 80% and 38%, respectively. The percentage of patients for whom no risk factor was identified (30% to 40%) did not change over time. These patients tended to belong to minority populations, and their socioeconomic level was low. The decline in the number of hepatitis B cases among homosexual men probably results from the modification of high-risk sexual behavior; the decline among health care workers is due mostly to hepatitis B immunization. The current strategy for prevention of hepatitis B, which targets high-risk groups for immunization, has failed to have a significant impact on the incidence of disease.