A 6-year-old boy with CML in blastic crisis was transplanted with BM and PBSC from his HLA-mismatched MLC-positive mother following CD34-positive selection. Preconditioning for transplant was with thiotepa, cyclophosphamide, rabbit anti-human thymocyte globulin, and TBI followed by infusion of 2.6 x 10(6)/kg of CD34-positive BM and PBSC. Engraftment was confirmed by FISH analysis, and GVHD was not observed. On day 50, he relapsed and died despite three transfusions of donor lymphocytes without GVHD prophylaxis. CD34-positive cell selection for HLA-mismatched transplantation may prevent severe acute GVHD.