Background:The importance of Chlamydia pneumoniae respiratory tract infection in childhood is under discussion. Patients and Methods: 798 hospitalized children with respiratory tract diseases were prospectively studied during a 2-year period by polymerase chain reaction and enzyme immunoassay (PCR-EIA) detection from throat swabs. Paired serum samples were used to screen for Chlamydia antibodies. Results: C. pneumoniae was detected by PCR-EIA in 74 children. Prevalence was 11% in lower and 4% in upper respiratory tract disease (p = 0.049) without age dependency. From November to February prevalence was elevated (42/277 vs. 32/521; p < 0.001). Using serology, prevalence of acute Chlamydia infection increased with age (p < 0.001) and the number of coinfections (p < 0.001), without seasonal variation.