The largest epidemic outbreak of legionnaires' disease occurred in Japan, where there were 45 cases, including three deaths. There was a high degree of homology at the genetic level between Legionella pneumophila sero-group 1 isolated from two patients and water samples collected from a public bath, and this highlighted the public bath visited by the patients as the source of the infection. We report our clinical observations of 34 cases with a pneumonia type (20 definite and 14 probable) of the 42 cases for whom a diagnosis was made and follow-up care was subsequently provided at our hospital or hospital-affiliated facilities, out of the initial total of 45 cases. Twenty-one cases had underlying diseases, but the disease was mild in all patients. Three cases showed different neuropsychiatric symptoms such as reduced capacity for memorization, somnolence or delirium. Diarrhea was found in three cases, and relative bradycardia in ten. Clinical laboratory test results indicated that a severe inflammatory reaction, renal or hepatic dysfunction, an electrolyte abnormality and abnormal urinalysis were common in the 34 cases. Chest radiography showed a shadow suggestive of infection of the pulmonary parenchyma with multilobar involvement in most cases and pleural effusion in nine cases.