The incidence of rotavirus gastroenteritis in infants and children that required admission to the hospital was estimated for a defined population of approximately 105,000 individuals, including 29,000 children aged 15 years or younger whose primary health care was provided by Group Health Association, Inc, a health maintenance organization in the Washington, DC, area. From January 1977 through March 1979, almost all infants and children in this age group who required hospitalization for gastroenteritis were studied for evidence of infection with human rotavirus (HRV) and other agents. On the average, one in 272 (3.7/1,000) infants less than 12 months old and one in 451 (2.2/1,000) children aged 13 through 24 months were hospitalized for HRV disease each winter. The incidence of rotavirus gastroenteritis requiring hospitalization declined precipitously in children after the second birthday and such illness was not detected in children aged 5 years or older. The role of other agents in acute gastroenteritis requring hospitalization was minimal, compared with that of rotavirus.