During 29 months of prospective longitudinal study of diarrhea in the home, human rotaviruses (HRVs) infected one or more members in 51% of 65 families, 35 of 126 children (28%) and 16 of 124 adults (13%). Within the 33 affected families, 57% of 62 children and 25% of 65 adults were infected. HRV gastroenteritis peaked at 40/100 person years at ages 12 to 23 months and decreased to 5 episodes/100 person years in adults. Among 25 children 0 through 36 months of age who had HRV infection, 88% were symptomatic. Of the 22 children with symptomatic HRV infection, 1 required hospitalization and 8 were seen by their physician for supportive care. HRVs were found in 12% of 216 stools obtained during gastrointestinal illness, but in only 0.2% of 1238 non-illness stools tested. HRV infections were noted as early as October and as late as April. Of 33 families who were studied for 2 seasons, at least 1 individual in each of 3 families experienced HRV infections in both years, but only one, an adult, shed virus and had symptoms in both seasons.