A 40-year-old man was admitted with high fever and cough. Pneumonic shadows of the left middle and lower lung fields increased rapidly, and his blood gases worsened. Initial treatment with cefmenoxime, piperacillin, and minocycline was ineffective. Administration of rifampicin was started for suspected legionella pneumonia, but it did not control the spread of the pneumonia shadows. After addition of an antifungal agent and trimethoprim-sulfamethoxazole, his symptoms gradually improved. Isolation of Legionella pneumophila from sputum specimens collected on the 4th day of admission confirmed the diagnosis on day 10. The patient was then given oral rifampicin plus cefmenoxime to prevent mixed infection, and showed a satisfactory improvement. Legionella pneumonia developed secondary to compromise of the patient's immunity due to steroid therapy for MDS. After recovering from Legionella pneumonia, the patient subsequently developed tuberculous pleurisy and Pneumocystis carinii pneumonia, which were cured by antituberculous therapy and trimethoprim-sulfamethoxazole. However, acute hepatitis followed by hepatic failure developed, and he died on day 121 after admission.