Although leptospirosis may be fatal in childhood, the experience of many clinicians working in disease-endemic areas is that classic Weil's disease and death are less common among pediatric patients. The aim of the study was to ascertain disease spectrum and outcome differences in severe pediatric and adult leptospirosis in a large at-risk population. Epidemiologic, clinical, and laboratory data were obtained on hospitalized cases from São Paulo during 2004-2006. A total of 42 case-patients <18 years of age and 328 case-patients ≥ 18 years of age were tested during the study. Compared with children, adults had higher rates of jaundice (P = 0.01), elevated serum bilirubin levels (P < 0.01), oliguria (P = 0.02), and elevated creatinine levels (P = 0.01) but not for thrombocytopenia or pulmonary involvement. The overall case-fatality rate was 27% (adult) versus 5% (pediatric) (P < 0.01). Severe pediatric leptospirosis may be less likely to show all classic features of Weil's disease and may be less fatal than in adults. Copyright © 2012 by The American Society of Tropical Medicine and Hygiene.