The nephrotoxicity of semustine (methyl-CCNU) has been studied in 45 adult patients with surgically resected Stage I or II malignant melanoma who received this drug as adjuvant chemotherapy. Abnormalities of renal function (including three cases of renal failure) were noted in seven of 45 patients (16 percent); all these patients received more than 1,400 mg/m2. This represents an incidence of 26 percent in patients receiving more than 1,400 mg/m2 of semustine. Two distinct patterns emerged. Abnormal serum creatinine levels developed in two patients while receiving semustine and later progressed to renal failure. Five patients had normal serum creatinine levels throughout their treatment courses but had abnormal creatinine values one month to two years following the completion of drug therapy. Renal failure developed in one of these patients, but the remaining four have had stable renal function for one to two years of additional follow-up. No clinical signs of renal insufficiency were detected in any patients receiving less than 1,400 mg/m2 of semustine. No changes unequivocally attributable to semustine were seen in eight patients at autopsy despite the fact that three had received greater than 1,900 mg/m2 of nitrosourea. This incidence of nephrotoxicity appears to be significantly lower than that previously reported in children. Guidelines for future therapy with semustine are described.