Bone marrow transplants were carried out in 18 patients with chronic myelogenous leukemia (CML) in the chronic phase (CP). 12 (67%) are still alive, 11 without evidence of leukemia after a mean observation period of 24 (3-44) months, 1 relapsed and 6 died. The most frequent cause of death was GvHD and interstitial pneumonia (5). 1 patient died of septicemia. 2 grafts were performed in patients with CML in the accelerated phase (AP); both died, one from leukemic relapse and one from GvHD. The authors also participated in an international study in which 117 patients were evaluated. In CP there was a survival plateau at 63%, in AP at 36% and in blastic crisis at 12%. In CP mortality was primarily age-dependent and relapses occurred in only 7%. It is concluded that bone marrow transplantation (BMT) is a highly successful treatment for CML, with the CP the optimum moment for grafting. Longlasting cytogenetic and clinical remissions with potential for cure are possible in a high percentage of patients. The incidence of transplant-related mortality is acceptable. The incidence of leukemic relapse is low in CP. Patients under age 40 with HLA-identical siblings should be transplanted in CP. At present BMT is the only treatment with curative potential for CML.