Reduced glutathione (GSH) has been reported to be an effective protector against cyclophosphamide-induced urotoxicity in experimental models, providing protection comparable to that of mesna. This paper describes our preliminary results of the clinical use of GSH in combination with cyclophosphamide. GSH was administered i.v. in two divided doses of 2.5 g, 15 min before and 30 min after escalating doses of cyclophosphamide ranging from 1.2 up to 1.6 g/m2 (1-h infusion). GSH was well tolerated and did not produce unexpected toxicity. The lack of bladder damage, including microscopic hematuria, supports the protective role of this thiol compound.