To determine the prognostic factors for leptospirosis, we conducted a retrospective study of data collected in the emergency department of our hospital between 1989 and 1993. Sixty-eight patients, for whom the diagnosis of leptospirosis was based on pertinent clinical and epidemiological data and positive serology, were included in this study. Fifty-six patients (82%) were discharged from the hospital, and 12 (18%) died. Multivariate logistic regression demonstrated that five factors were independently associated with mortality: dyspnea (odds ratio [OR], 11.7; 95% confidence interval [CI], 2.8-48.5; P < .05), oliguria (OR, 9; CI, 2.1-37.9; P < .05); white blood cell count, >12,900/mm3 (OR, 2.5; CI, 1.8-3.5; P < or = .01), repolarization abnormalities on electrocardiograms (OR, 5.9; CI, 1.4-24.8; P < or = .01), and alveolar infiltrates on chest radiographs (OR, 7.3; CI, 1.7-31.7; P < or = .01). Identification of these factors on admission might provide useful selection criteria for patients who need early transfer to the intensive care unit.