This study was undertaken to compare prospectively the frequency, nature, and severity of infections in children participating in three types of child care arrangements: home care, group care (two to six childre), and day care (seven or more children). Children were enrolled at birth and observed for at least 36 months. The families were telephoned every 2 weeks to record on a standardized form the type and severity of illness during the previous interval. Children remaining in their original child care group for 1, 2, or 3 years were compared with regard to the frequency and severity of illness. Each child care group had the highest number of infections in year 2. Children in day care had more respiratory infections during each year than children in home care, but the magnitude of the differences decreased in year 3. When the child care groups were compared for the proportion of children with more than six illnesses per year or more than 60 days of respiratory illness per year, significant differences observed in years 1 and 2 for day care children compared with home care children were no longer significant in year 3. We conclude that there is a trend toward stabilized or decreased rates of infection, duration of illness, and risk of hospitalization for children remaining in day care for 3 years.