Based on previous reports suggesting that an intravenous (i.v.) continuous infusion of alkylating agents produced a significant response rate in acute lymphoblastic leukemia (ALL), a phase I trial of ifosfamide/mesna (IFO/MES) was conducted in 11 adult patients with relapsed ALL. IFO/MES were administered as a 24-h i.v. continuous infusion in doses ranging from 5 g/m2 to 9 g/m2; the courses of treatment were repeated every 3 weeks. Patients were examined for toxicity after every cycle and responses were carefully defined and evaluated. All 11 admitted patients were evaluable for toxicity and response. Myelosuppression was the dose limiting effect and it was dose-related. Microscopic and/of macroscopic hematuria was detected in four (11%) out of 36 cycles administered. Ifosfamide produced a positive biological response with a preliminary response rate of 45.4%. Ifosfamide/mesna in a 24-h i.v. continuous infusion appears to be tolerated and produces a biological response in ALL. We recommend that phase II studies of this drug schedule be conducted at the initial dose of 7 g/m2 repeated every 3 weeks and combined with other antileukemic agents.