A cohort of 45 children was observed from birth to three years of age in their natural ecosystem to determine patterns of infection, morbidity, and growth. Data from enzyme-linked immunosorbent assay analysis for rotavirus of 5,891 extracts (kept frozen since 1964-1969) of weekly fecal specimens were compared against growth, morbidity, and specimen data files, permitting a retrospective description of the epidemiology of rotavirus infection in the cohort. Rotavirus infections were uncommon in the first months of life in intensively breast-fed infants. Infection increased with age to reach a maximal rate in the six- to 18-month age period. While there was a high incidence of diarrhea in the cohort, rotavirus was associated with only 10% of such episodes. The incidence of rotavirus infection was 1.2 episodes per child-year, and the incidence of rotavirus-associated diarrhea was 0.8 episodes per child-year. Serious outbreaks of rotavirus generally occurred from September through December, with as many as one-half of the children becoming infected. Repeated rotavirus infection was a common phenomenon.