Acute renal insufficiency developed in four idiopathic nephrotic patients with minimal change or mild proliferative glomeruionephritis. The reduction in glomerular filtration rate (Qnuiin) was not m Proportion to the renal plasma flow (CPAH) as evidenced by a low filtration fraction. Diuretic therapy failed to reverse renal insufficiency, and renal biopsy showed no evidence of interstitial nephritis, acute tubular necrosis or interstitial edema. Corticosteroid therapy induced a recovery of renal function with a decrease in proteinuria. These observations suggest that acute renal insufficiency in the idiopathic nephrotic syndrome might be caused by impaired glomerular permeability. (Internal Medicine 32: 31-35, 1993)