We investigated 56 boys and 30 girls between the ages of 1 month and 15 years with acute respiratory infections for antibodies toChlamydia pneumoniaeandMycoplasma pneumoniae. Antibodies were only occasionally found in children less than 10 years of age (4% of boys (1/23) and 5% of girls (1/19). In the age group 10–12 years, antibodies were detected in 55% of boys (4/9) but not in the 2 girls investigated. In the age group 13–15 years, antibodies were detected in 58% of boys (14/24) and in 33% of girls (3/9). Infection caused byM. pneumoniaewas found in 13% of boys in the oldest age gro