Acute renal insufficiency of various etiologies is presently associated with high mortality rates in spite of modern methods of treatment.1 The possibility of preventing its occurrence rightly commands our attention. The initial reports of Barry and his colleagues2,3 suggested that the osmotic diuretic, mannitol, exerted a protective action on the kidney when given prior to, and during, aortic cross clamping above the renal arteries. A large number of papers followed claiming that the incidence of acute renal insufficiency is reduced when mannitol is given promptly in situations varying from hemorrhagic shock to poisoning with nephrotoxic chemicals. Many of these appear in the proceedings of a symposium held in 1962.4 Others are in the form of isolated case reports.5 Although much of the animal data seems encouraging,4,6 evaluation of the clinical studies has been hampered by the low incidence of acute renal insufficiency and the difficulty