Leptospirosis is a common disease in Latin America. Transmission to humans occurs by contact with water or soil contaminated with the urine of rodents, dogs, or livestock. Pathogenesis is still poorly understood, and bacterial toxin or virulence factors are probably responsible for many features of the disease. The anicteric form is the most frequent presentation, and its clinical picture resembles influenza or other acute febrile diseases. Icterohemorrhagic leptospirosis, or Weil's syndrome, represents the severe form of the disease. Its clinical picture is similar to bacterial sepsis and multiple organ involvement occurs, mainly in kidneys and lungs, and causes great morbidity and mortality. Death is often related to multiple organ failure and pulmonary hemorrhages. Diagnosis is based on serology or blood, cerebrospinal fluid and urine cultures in specific media. Treatment involves a combination of antibiotics and supportive measures.