This study was undertaken to compare prospectively the frequency, nature, and severity of infections experienced by children in three types of child care arrangements: home care, group care (two to six children), and day care (seven or more children). Children were enrolled at birth and observed for 12 to 18 months. At entry there were 159 children in home care, 40 in group care, and 45 in day care. The families were telephoned every 2 weeks to record on a standardized form the type and severity of illnesses experienced during the previous interval. Severe illnesses were defined by high fever, duration exceeding 10 days, or physician visit. Children remaining in their original child care group for at least 1 year were compared with regard to the frequency and severity of illness. Children in group care and day care were more likely than children in home care to experience at least six respiratory infections, more than 60 days of illness, and more than four severe illnesses (P less than 0.01). Similarly, life table analyses showed that children in home care had fewer episodes of infection than did children in day care (P less than 0.01). Although no children were hospitalized because of acute infections during the first year of study, hospitalization for myringotomy and tube placement occurred in 21% of children in day care and 3% of children in home care (P less than 0.01).