A 64-year-old male consulted our hospital with a 6-day history of malaise, body temperature to 38 degrees C, anorexia, and light headedness. The chest radiograph showed consolidation in the left lower lung area. Chest CT and ultrasonography revealed left pleural fluid. The pleural fluid was exudative (yellowish in color, protein 3.0 g/dl, Rivalta reaction positive, adenosine deaminase 19.4 U/L), and had a total cell count of 4.7 x 10(6)/ml with 45% lymphocytes, 40% histiocytes, and 15% polymorphonuclear leukocytes. He had kept a budgerigar, but we could not isolate Chlamydia from the pleural fluid or the pet bird. Transbronchial lung biopsy from left S10 revealed an increase of mononuclear leukocytes within the interstitial spaces, and the alveolar spaces contained polymorphonuclear leukocytes, fibrin, and organized alveolar exudate. Bronchoalveolar lavage cellular constituents were 50% lymphocytes, 27% neutrophils, and 23% macrophages. Serologic studies demonstrated C. trachomatis specific IgM antibody titers at 1:20 in a serum sample and at 1:10 in pleural fluid. We report a case of community-acquired pneumonia caused by C. trachomatis diagnosed by serologic studies.