A 53-year-old male was admitted to Keio University Hospital with a pneumonia shadow in the left lung field and respiratory failure. Because there was progression of respiratory failure, mechanical ventilation was required to maintain appropriate oxygenation. Although erythromycin administration was started at the time of admission, a steroid (prednisolone 60 mg/day) was added a few days later to temporarily inhibit the acute inflammatory response in the lung parenchyma. This intensive therapy resulted in resolution of the patient's pneumonia and improvement of his respiratory failure. No pathogens were detected in the clinical specimens. Indirect immunofluorescence examination demonstrated a marked increase in titers against Legionella pneumophila serogroup 1, which was sufficient to confirm a diagnosis of Legionnaires' disease. The causative organism of this disease, a gram-negative short rod, is rarely cultured on conventional culture media. Two clinical subtypes are known based on clinical manifestations: 1) the Pontiac fever-type in which the predominant symptom is fever alone; and 2) the pneumonia-type which was observed in the epidemic in Philadelphia when the disease was first reported in 1976. The present case of Legionnaires' disease was the severe pneumonia-type which was successfully treated with a combination of erythromycin and a steroid.