Streptozotocin (STZ) has shown antitumor activity against various tumors in man, but the clinical usefulness of this drug has been limited, mainly because of renal and gastrointestinal toxicity. Nineteen patients with advanced cancer of various types were given a mean dose of 3.4 g/m2 of STZ by continuous iv infusion over 5-6 days each month for one or two monthly cycles. Basic serum and urine studies were performed immediately before and after each treatment cycle. Following STZ treatment, no significant changes in BUN or creatinine were seen. Four patients in whom initial tests for proteinuria were negative developed grade 1 or 2+ proteinuria after completion of the treatment cycle. No myelosuppression or renal failure was observed. Six patients had no nausea or vomiting, seven patients had nausea only, three patients had nausea and vomiting which were well-controlled with antiemetics, and three patients had uncontrollable nausea and vomiting. Confusion, lethargy, and depression were noted in five patients who had no prior central nervous system abnormalities; these effects appeared during treatment or in the immediate posttreatment period. Two patients with diffuse non-Hodgkin's lymphoma had complete remission, while several other patients had documented improvement. Although central nervous system toxicity may be a limiting factor, prolonged STZ infusions may have significant clinical promise.