HLA-identical sibling bone marrow transplantation is an effective and commonly used therapy for young patients with chronic myelogenous leukemia. Efficacy results from high dose chemotherapy, with or without radiation, given for pretransplant conditioning and from immune-mediated antileukemia effects of the graft. The primary determinant of outcome is the patient's disease phase at time of transplant, with best results observed when transplants are done early in the chronic phase. Major causes of treatment failure are graft-versus-host disease and other transplant-related complications. Relatively few patients relapse unless the disease is advanced pretransplant or the donor bone marrow is T-cell depleted.