Leptospirosis is the disease produced by any of the group of spirochetes of the genus Leptospira. The main organs involved are the liver, central nervous system, kidneys, skeletal muscle, and lungs. Thirty-seven (64%) of 58 patients with leptospirosis, proved by positive serology, had pulmonary radiographic findings. Three radiographic patterns were evident: (1) 21 (57%) of the 37 patients had small nodular densities, (2) six (16%) had large confluent areas of consolidation, and (3) 10 (27%) had diffuse, ill-defined, ground-glass density. Serial radiographs showed a tendency for the nodular pattern to be followed by confluent consolidation and/or ground-glass density. Abnormalities were bilateral, nonlobar in all cases, and had a marked tendency toward peripheral predominance. Pulmonary abnormalities resolved within 15 days, except in eight patients who died because of respiratory failure (six patients) or other causes (two patients). In order to correlate pathology with the radiographic findings, Leptospira, isolated from a patient, was injected intraperitoneally into 20 guinea pigs. All lungs from the guinea pigs showed petecheal hemorrhage, which progressed to large confluent areas of hemorrhage. The typical pulmonary radiographic findings of leptospirosis are compatible with the multifocal pulmonary hemorrhage seen in the guinea pigs.