Renal involvement is common in leptospirosis. Clinical manifestations vary from urinary sediment changes to acute renal failure. Renal failure is observed in 44% to 67% of patients. Hypokalemia frequently occurs. Severe hypotension is an important warning sign for the later development of renal and pulmonary complications. Prognosis of the disease is generally good except for its association with pulmonary complications, especially pulmonary hemorrhage and acute respiratory distress syndrome. Interstitial nephritis is the basic renal lesion. Vasculitis is observed in the acute phase of the disease. Tubular necrosis and interstitial nephritis are responsible for renal failure. Glomerular changes usually are not remarkable. Hemodynamic alterations, immune response, and direct nephrotoxicity are responsible for the development of renal lesions. As in many infectious diseases, decreased renal blood flow and glomerular filtration rate play a basic role. Bacterial invasion and toxicity of outer membrane with generation of cytokines, chemokines, and cellular infiltration are important in cellular injury.