Fourteen patients with chronic myeloid leukemia (CML) relapsing after allogeneic bone marrow transplant (BMT) were treated with leukocyte transfusions from the original marrow donor (sibling, n = 9; volunteer unrelated, n = 5). The relapse was defined at the molecular level in two cases, cytogenetically in five cases and hematologically in seven cases. Ten patients responded, seven of seven patients with cytogenetic/molecular relapse compared with three of seven with hematologic relapse (P < .03). All five recipients of cells from unrelated donors responded. Eight of the 10 responders have achieved polymerase chain reaction-negative status and this persisted in three patients for more than 2 years; no responder has shown sign of relapse. Reversible marrow aplasia occurred in two patients, both treated in hematologic relapse. Severe graft-versus-host disease occurred in four patients and was fatal in one. We confirm previous reports that donor leukocyte transfusions are effective in the management of CML in relapse after BMT. In this series, therapeutic intervention before the onset of hematologic relapse was associated with an increased likelihood of response and no marrow aplasia.