The objective of this study was to assess the incidence of pulmonary involvement in a cohort of 26 patients in whom a diagnosis of leptospirosis had been made. Seventeen of the 26 patients had respiratory symptoms. Of these 17 patients, 13 had radiographs showing pulmonary abnormality. The most frequent finding was a bilateral patchy alveolar-acinar pattern in six patients. Three patients developed acute respiratory distress syndrome and died due to multiorgan failure. Only cigarette smoking was significantly associated with respiratory involvement (odds ratio, 19.2; 95% CI, 1.7-250; P < 0.001). The results indicate that pulmonary manifestations are observed in a high percentage of patients with leptospirosis. Cigarette smoking is a risk factor for the development of pulmonary involvement in human leptospirosis.