A 72-year-old man was admitted to our hospital because of acute respiratory failure. Initially he had been diagnosed as having pneumonia in the lower left lung field and treated with cephem antibiotics by a local physician. Chest X-ray photograph revealed wide-spread infiltrates throughout both lungs, and chest CT scan revealed pleural effusions. The partial pressure of oxygen in arterial blood was 45.8 Torr. In the bronchoalveolar lavage fluid (BALF) specimen, legionella DNA was detected by the polymerase chain reaction (PCR) method. Thus, we were able to diagnose Legionella pneumonia immediately, and to treat the patient successfully.