Transplantation of marrow from unrelated donors was investigated in patients with Philadelphia chromosome-positive (Ph1+) acute lymphoblastic leukemia (ALL) who lacked a suitable family donor. Eighteen patients underwent transplantation at our center between 1988 and 1995. The median patient age was 25 years (range, 1.7 to 51 years). Seven patients were in first complete remission, 1 in second remission, 3 in first relapse, and the remaining 7 had more advanced or chemotherapy refractory leukemia at transplant. All patients were conditioned with cyclophosphamide and total body irradiation followed by marrow transplants from closely HLA-matched, unrelated volunteers. Posttransplant graft-versus-host disease (GVHD) prophylaxis included methotrexate with either cyclosporine or FK506. Graft failure was not observed. Severe (grades III-IV) GVHD appeared in 6 of 17 evaluable patients and chronic extensive GVHD in 7 of 13 patients at risk. Five patients had recurrent ALL after transplantation and another 4 died from causes other than leukemia. Six patients transplanted in first remission, 2 in first relapse, and 1 in second remission remain alive and leukemia-free at a median follow-up of 17 months (range, 9 to 73 months). The probability of leukemia-free survival at 2 years is 49% +/- 12%. These data indicate that unrelated donor marrow transplantation is an effective treatment option for patients with early stage Ph1+ ALL without a family match and suggest that in such patients an unrelated donor search should be initiated as soon as possible after diagnosis.