Fourteen children between the ages of 2 and 5 years with juvenile chronic myelogenous leukemia were given cyclophosphamide, total-body irradiation, and marrow transplants. Unmodified marrow was given to six patients who received marrow from HLA-identical siblings and eight patients who received marrow from family members HLA identical for one haplotype but mismatched for one to three loci on the nonshared haplotype. Five patients died of transplant-related complications, and three relapsed at 48, 81, and 1,670 days posttransplant and died of leukemia. Six patients survive in continuous remission from 0.5 to 11.5 years posttransplant.