We assessed clinical, laboratory and radiologic findings in 25 patients (21 men and 4 women; mean age 60.1 years) with Legionella pneumonia. The diagnosis was confirmed from the isolation of Legionella pneumophila in respiratory samples(nine cases), or by a fourfold increase or decrease in antibody titers in the paired serum samples (16 cases). Twelve patients were smokers, 5 patients were chronic alcoholics and 18 patients had underlying diseases. Regarding severity, the illnesses were mild (three patients), moderate (seven patients) and severe (fifteen patients). The following presenting symptoms and findings that characterize Legionella pneumonia were seen: a temperature of more than 40 degrees C (six patients), disorientation or gate disturbances(ten patients), diarrhea (eight patients), relative bradycardia(eight patients), hyponatremia(five patients), and a high creatinine phosphokinase suggesting rhabdomyolysis(five patients). Rapid progression of infiltrates on chest radiographs before appropriate antibiotic therapy was observed in seven of the twelve patients whose previous chest films were available. Only one patient, who was in endotoxin shock on admission, died. Suspected sources of infection for 13 patients were a hot spring and a public bath. We conclude that a detailed analysis of the above clinical features makes possible the diagnosis of Legionella pneumonia.