BACKGROUND: The epidemiology of Chlamydia pneumoniae in healthy children has not been established. METHODS: This investigation used polymerase chain reaction (PCR) to study the epidemiology of C. pneumoniae in presumed healthy children. Four hundred fifty-three children and 142 personnel at 9 day-care centers were investigated for carriage of C. pneumoniae. Children found to be positive by PCR were also investigated with serology, and their family members were tested with PCR. RESULTS: One hundred and three (22.7%) children had a positive PCR, as had 33 (23.2%) personnel. Fourteen percent of the children younger than 3 years had a positive PCR test compared with 26% of the older children (P