A 52-year-old man was admitted to our hospital because of fever and dyspnea. Chest X-ray film showed infiltrative shadows in the right lower lung fields and serum creatinine was 6.7 mg/dl. The patient was diagnosed as pneumonia complicated with acute renal failure. He was treated with antimicrobial therapy. However he developed respiratory failure and exhibited mental confusion. Legionnaires' disease was suspected, because pneumonia was found to be associated with multiple organ dysfunction. We started treatment with intravenous administration of minocycline and continuous hemodiafiltration and his condition had been rapidly improved. We diagnosed this case as legionnaires' disease due to the positive of urinary Legionella pneumophila serogroup 1 antigen. Since the cases of legionnaires' disease with acute renal failure are serious, prompt diagnosis and adequate treatment are essential.