Clinical and pharmacokinetic studies were performed regarding the toxicity of methyl 6-[3-(2-chloroethyl)-3-nitrosoureido]-6-deoxy-alpha-D-glucopyranoside (MCNU) with other drugs, in conjunction with a peripheral blood stem cell autograft (PBSCT), for treating 26 children with acute leukemia or lymphoma associated with high-risk features. In the early phase of the study, MCNU (300 to 500 mg/m2) was administered with cytosine arabinoside (Ara-C) (1.6 to 16 g/m2), etoposide (VP-16) (0.8 to 1.6 g/m2), cyclophosphamide (CY) (100 to 200 mg/kg), or busulfan (16 mg/kg). No acute toxicity was noticed after this high-dose therapy. The dose-limiting factor of the regimens was significant but reversible interstitial pneumonitis (IP). In a subsequent trial with an MCNU/VP-16/Ara-C/CY (MCVAC) regimen in which the dose of MCNU was reduced, the risk of IP diminished. This study is still in progress, but the clinical response has so far been encouraging. Fifteen of 26 children are alive and well in unmaintained complete remission (CR) with a median follow-up period of 11 months (range, 3 to 34 months) after transplantation. This MCNU-based regimen without total body irradiation (TBI) is especially important in children to avoid the serious sequelae of irradiation. Our results justify a broader clinical trial to evaluate the effects of the MCVAC regimen followed by PBSCT.