We prospectively investigated the etiology of community-acquired pneumonia among adult patients admitted to three general hospitals over one year. Antibody titers to Chlamydia pneumoniae were also measured by ELISA method to investigate the incidence and clinical pictures of C. pneumoniae pneumonia in Japan. Two hundred and fourteen patients were enrolled in the study. Seventeen patients (7.9%) satisfied the criteria for diagnosis of acute infection due to C. pneumoniae by ELISA method. C. pneumoniae was the 3rd leading causative agent following Streptococcus pneumoniae (21.5%) and Haemophilus influenzae (8.4%). Preexisting antibodies were detected in 62.1% of the patients. Most of the patients with C. pneumoniae pneumonia were aged, and had comorbidities of respiratory system. Superinfection with bacterial pneumonia was recognized in some patients. Though there are several problems, ELISA method is thought to be useful to diagnose C. pneumoniae pneumonia, and is suitable for screening tests.