The renal profile was studied in 61 patients having leptospirosis. Leptospira-induced acute renal failure conformed to one of two distinct clinical patterns, either oliguric on non-oliguric renal failure. The prognosis was excellent in the latter variety of renal failure and mortality was confined to patients with oliguric renal failure, where severe glomerular and widespread tubulo-interstitial lesions were invariably present. These patients had unfavourable prognostic features during life, such as prolonged oliguria and anuria, absence of a diruetic phase, persistent elevations in BUN and persistently low urea excretion. The prognostic significance of the age of the patients, hypotension and jaundice in relation to the occurrence and type of acute renal failure have been discussed. Retrospective observations indicate that both a vasculotoxic or haemorrhagic state and oliguric acute renal failure are important causes for mortality in human leptospiral infections.