Semustine is an investigational cancer chemotherapeutic agent in widespread use. This agent has now been documented to produce nephrotoxicity and renal failure with long-term administration. We collected 29 cases of semustine nephrotoxicity from the literature and six unpublished cases brought to the attention of the National Cancer Institute. Using these 35 cases as a data base, we have analyzed the incidence, dose and treatment duration relationships, clinical and histologic manifestations, and clinical course of semustine nephrotoxicity. In addition, we discuss the possible mechanisms of this nephrotoxicity based upon ongoing laboratory work. We conclude that there is a high risk of severe nephrotoxicity from semustine when the cumulative dose exceeds 1200 mg/m2 and that there may be considerable delay in onset of the renal dysfunction.