Acute organic renal failure is preceded by a reversible functional failure in most patients. Prompt mannitol therapy reversed functional and prevented total organic failure in 16 of 24 patients treated. A tentative diagnosis of organic failure was based on the findings of prolonged oliguria, low urinary specific gravity, azotemia without azoturia, inappropriate urinary sodium excretion, and an abnormal urinary sediment. The acute intravenous infusion of 12.5 gm. mannitol was supplemented in 13 of the 16 responsive patients to maintain urine flow at 100 ml. per hour; thus, abnormalities of water balance, electrolyte balance, and acid-base balance associated with oliguric renal failure were circumvented. Increasing azotemia and other evidence of organic failure persisted for several days in some patients, but none developed clinical uremia. Mannitol infusion appeared to be a life-saving measure in many patients.