Specific IgG and IgM antibodies to Chlamydia pneumoniae were evaluated by microimmunofluorescence test in a group of healthy subjects (age 0-18 years). Antibody titers (IgG) > or = 1:16 were found in 9.7% of the age group 0-1 years, in 5.3% of the group of 2-3 years and rose to 19% in children of the group 4-6 years. The seroprevalence was 11% and 17.1% in the group 7-12 years and 13-15 years, respectively. In the last group of 16-18 years the seroprevalence was 9.1%. The overall seroprevalence was 11.8% and C. pneumoniae infection is acquired before school age. In addition, in our pediatric patients it seems that C. pneumoniae does not play an important pathogenic role, being responsible for only 1.6% of the acute lower respiratory infections. The factors that may account for these findings are discussed.