We prospectively studied the etiology of community-acquired pneumonia among all patients who were admitted to our hospital from July 1994 to June 1995. Tests for microbial pathogens including Chlamydia spp. and Legionella spp. were performed and diagnoses were made with strict criteria. A total 110 patients with 111 episodes of pneumonia were evaluated, and a pathogen was identified in 61 episodes (55%). The most common pathogen was Streptococcus pneumoniae (18%), followed by Haemophilus influenzae, Klebsiella pneumoniae, Pseudomonas aeruginosa, Mycoplasma pneumoniae, and Chlamydia spp. Infection with Legionella pneumophila was not found. Dual pathogens were identified in five episodes. Few prospective studies of the etiology of community-acquired pneumonia have been done in Japan. To prepare guidelines for the management of community-acquired pneumonia in Japan, a national study of the etiology of pneumonia is necessary.