Eighteen chronic myeloid leukemia patients with hematological (four patients) or only cytogenetic (14 patients) relapse occurring after T cell-depleted allogeneic bone marrow transplantation (BMT) have been treated with alpha 2b interferon (IFN) at a starting dose of 5 x 10(6) i.u./m2 subcutaneously three times a week. All four patients with hematological relapse achieved long-lasting hematological remission without reduction of bone marrow Ph1 positive cells. When IFN was started the median percentage of bone marrow Ph1-positive metaphases was 50% (range 9-100) for the 14 patients with cytogenetic relapse. Twelve (85.7%) of these patients are alive with a median follow-up of 25 months (range 20-37 months) from cytogenetic relapse and 33 months (range 27-49 months) from BMT. Six (43%) of the 14 patients progressed to hematological relapse and eight patients (57%) are still in hematological remission with two patients achieving complete cytogenetic remission confirmed at molecular level by disappearance of the M-BCR rearranged band. IFN therapy may be a good alternative to conventional chemotherapy for transplanted CML patients with hematological relapse and the treatment of choice for patients with a persistent cytogenetic relapse occurring after T cell-depleted BMT.