We evaluated a total of 1104 pediatric patients with acute lower respiratory tract infection for C. pneumoniae infection and M. pneumoniae infection by serology during July 1995 to December 1998. A microimmunofluorescence test was used for diagnosis of C. pneumoniae infection and a high density particle agglutination test for that of M. pneumoniae infection. Acute C. pneumoniae infection was found in 149 patients (13.5%), acute M. pneumoniae infection in 118 patients (10.7%), and dual infection in 27 patients (2.4%). Among 305 patients with pneumonia, M. pneumoniae infection (83 patients, 27.2%) was more common than C. pneumoniae infection (47 patients, 15.4%). However among 799 patients with bronchitis. C. pneumoniae infection (102 patients, 12.8%) was more common than M. pneumoniae infection (35 patients, 4.4%). Patients with C. pneumoniae infection were more younger and more frequently wheezing than patients with M. pneumoniae infection. These findings demonstrate that C. pneumoniae infection in very common pathogen of pediatric lower respiratory tract infection as M. pneumoniae infection in Japan.