Human leptospirosis of the classical and severe icterohemorrhagic type, usually due to the L. icterohaemorrhagiae serogroup, is frequent in La Réunion. In a retrospective study conducted between 1980 and 1984 in 249 adult patients, the mortality rate was 13 p. 100. Our data and those found in the literature indicate that the main cause of death is pneumopathy, followed by profuse haemorrhages, arrhythmias and cardiovascular collapse. Acute renal failure is common and often severe; it facilitates gastrointestinal bleeding and is of poor prognosis, particularly in patients with prolonged anuria, a possible cause of lethal hyperkalaemia. Other factors of unfavourable outcome have been demonstrated statistically; they include disturbances of consciousness, hypoprothrombinaemia, epigastric muscle rigidity, hyperleukocytosis, thrombocytopenia, high aspartate aminotransferase levels and chronic alcoholism. At the moment, pulmonary, cardiac and haemorrhagic complications concur with renal failure to darken the prognosis of these severe forms of leptospirosis.