Eight patients with chronic myeloid leukemia relapse after allogeneic BMT were treated with IFN-alpha and buffy coat transfusions (BC) of the bone marrow donor. The antileukemic effect of this treatment was directly demonstrated in 4 patients by the disappearance of Philadelphia chromosome-positive metaphases or the loss of detectable BCR-ABL transcripts by polymerase chain reaction. In 2 patients in whom cytogenetic or polymerase chain reaction analysis was not performed, a change in hemopoietic chimerism with recurrence of donor-type hemopoiesis was demonstrated. Two patients, both treated in advanced stages of hematological relapse after BMT, did not respond. However, severe side effects of the treatment were observed: graft-versus-host disease (GVHD) occurred in 5 patients. Two of these patients progressed to severe chronic GVHD and 1 patient ultimately died of this complication. GVHD occurred in 5 of the 6 responding patients; one patient responded without developing clinical symptoms of GVHD. Six patients developed bone marrow hypoplasia after IFN/BC treatment, and pancytopenia occurred in 4 patients. None of these 4 patients recovered spontaneously and 2 patients died of complications of pancytopenia (cerebral bleeding, infection). Our results demonstrate that treatment of chronic myeloid leukemia relapse with IFN and BC transfusions is highly effective in patients with relapse in chronic phase. The occurrence of GVHD and pancytopenia, however, resulted in a high treatment-associated morbidity and mortality. Whereas a response to treatment was observed in 1 patient without GVHD, indicating that GVHD and a graft-versus-leukemia effect may be clinically separable, bone marrow hypoplasia occurred in all responding patients.