Renal insufficiency, with serum creatinines ranging from 2.3 to 13.4 mg/dl, was observed in 15 patients with the minimal change nephrotic syndrome. Recovery of renal function occurred in association with diuretic therapy in 13, eight of whom subsequently underwent steroid-induced remission of the nephrotic syndrome. Two patients failed to undergo diuresis or to have a remission of the nephrotic syndrome and died with persistent renal failure. Glomerular filtration rate (Cinulin) was reduced out of proportion to renal plasma flow (CPAH) as evidenced by remarkably low filtration fractions ranging from 0.03 to 0.095. The invariable association between diuresis and recovery of renal function, the recurrence of renal failure when fluid reaccumulated and the finding of markedly depressed filtration fractions lead us to postulate that renal failure in minimal change nephrotic syndrome may be due to a reversible alteration in glomerular hemodynamics which is related to fluid retention and associated intrarenal edema.