From July to August 2002, a large outbreak of Legionnaires' disease occurred in the northern area of Miyazaki Prefecture, Japan. Since the genotype of Legionella pneumophila serogroup 1 found in the water of a new public bathhouse was identical to the isolates obtained from the sputum of the patients, the bathhouse was identified as the source of the outbreak. We report the clinical characteristics of 37 pneumonia patients (9 confirmed and 28 probable cases, including one death). Ten patients had diabetes mellitus and nine were smokers. Almost all had high fever, and nine patients had a non-productive cough. In regard to extrapulmonary symptoms, neuropsychiatric symptoms were found in nine cases and abdominal symptoms in 12. Laboratory tests revealed that a severe inflammatory reaction, liver dysfunction, rhabdomyolysis and hyponatremia were common. Chest radiography demonstrated that about half of the patients had bilateral shadows and the alveolar filling pattern was common to almost all patients.