A 60-year-old diabetic man who had had a coronary artery bypass graft operation was admitted to Okaya Enrei Hospital because of coughing, high fever and dyspnea. Chest high-resolution computed tomography scans revealed bilateral pleural effusions and left-sided alveolar shadows and ground glass opacity. These infiltrations in the left lung field showed rapid growth. Legionella pneumonia was diagnosed because of a high titer for Legionella pneumophila antigen in the urine. He was treated with 600 mg per day of parenteral ciprofloxacin for two weeks and 10 mg per day of oral prednisolone for the second week, resulting in improvement of the clinical findings.