We conducted a retrospective analysis of 43 consecutive children (35 boys and 8 girls), 4-14 years of age and living in an urban area, who were hospitalized at the Instituto de Infectologia Emilio Ribas (Sao Paulo, Brazil) from January 1989 to December 1995 with an acute illness subsequently diagnosed as leptospirosis. Epidemiologic data indicated contact with contaminated water in most cases (88%). The patient sera reacted most strongly with Leptospira interrogans serovars copenhageni (45%) and icterohaemorrhagiae (32.7%). Jaundice was present in 70% of the patients, elevated transaminase levels in 56%, renal failure in 79%, meningitis in 23%, thrombocytopenia in 65%, and hemorrhagic manifestations in 11.6%. Three children had pulmonary hemorrhage with respiratory failure and one death occurred as a consequence of respiratory failure. We also observed that antimicrobial therapy reduced the extent of renal failure and thrombocytopenia. These data indicate that antibiotics benefit children with late, severe leptospirosis and that severe disease also occurs in children and should be considered in the differential diagnosis.