To determine whether well children visiting a pediatrician's office are at risk of acquiring common communicable diseases, we conducted a prospective cohort study during the winter of 1983-1984. The occurrence of respiratory or gastrointestinal illness or fever in young children in the week after a visit to a pediatric office (office group) was compared with the occurrence of these illnesses in a similar group of children (home group) not exposed to the office during the study period. One home comparison subject was matched by age and sex to each of 127 children three years of age and under who were seen for well-child care in a private pediatric office with a common waiting room. The adjusted relative risk of the development of illness during the study week for the office group as compared with the home group was 0.95 (95 per cent confidence interval, 0.66 to 1.37), indicating no effect. We conclude that exposure to a pediatric office is not an important cause of the common infections in young children that have incubation periods of a week or less.