A 67-year-old male was admitted to our hospital due to a high fever with abnormal shadows on chest X-ray films. On admission, his laboratory data showed hyponatremia, rhabdomyolysis and liver dysfunction. Encephalopathy, acute renal failure and respiratory failure developed, despite fluid management and antimicrobial therapy. His condition worsened rapidly in a few days enough to require mechanical ventilation. Legionnaires' disease was suspected, because pneumonia was found to be associated with multiple organ dysfunction. Intravenous erythromycin and methylprednisolone were administered. The patient's condition was rapidly improved, although he needed hemodialysis for 30 days. Later, indirect fluorescent antibody testing of the patient's serum against Legionella pneumophila was definitely positive (1:1024). We reported the first case of severe Legionnaires' disease in Miyazaki Prefecture, Japan.