We report two cases of acute renal failure in patients with arteriosclerosis obliterans treated by intravenous infusion of naftidrofuryl oxalate. At renal biopsy the histological lesions were identical with those found in ARF due to hyperoxaluria of other causes, revealing tubular epithelial necrosis and massive intratubular precipitation of calcium oxalate monohydrate (C1) crystals. A second study was then conducted in four other patients with arteriosclerosis obliterans to evaluate serum and urinary levels of oxalate, and crystalluria during the intravenous administration of 800 mg of naftidrofuryl oxalate per day for 10 days. During the course of treatment, the serum and urinary oxalate levels were found to increase substantially, with the gradual onset of massive C1 crystalluria. These results indicate that naftidrofuryl oxalate was responsible for the acute renal failure in the first two patients. High intravenous doses of naftidrofuryl oxalate must be used cautiously, with close surveillance of renal function.