Eighteen patients with relapse of chronic myelogenous leukemia (CML) after allogeneic bone marrow transplantation (BMT) were treated with recombinant human alpha 2a interferon (IFN). Relapse was defined as greater than 90% metaphases containing the Philadelphia chromosome (Ph) and hematologic abnormalities consistent with chronic-phase (CP) CML. There were 11 males and seven females, with a median age of 38 years (range, 3 to 55). Three patients relapsed after second BMT. Only one patient had received T-cell-depleted marrow initially. The initial IFN dose of 3 x 10(6) U/m2/d was escalated to the maximum tolerated dose or to a maximum of 6 x 10(6) U/m2/d. IFN controlled the white blood cell (WBC) counts in 14 of 16 patients who had abnormal counts, and in all six patients with an elevated platelet count. Six patients (33%) have had a complete disappearance of the Ph and two have had a partial response (less than 35% Ph+ metaphases). One patient has a decrease in Ph+ metaphases after 9 months of IFN. Five patients had no significant cytogenetic response after 9 to 12 months, and four developed clinical accelerated phase or blast crisis after 3 to 6 months on therapy. Of four patients with a sex marker, the Ph- population was of donor origin in three and of host origin in one. Clonal cytogenetic abnormalities other than Ph were present in 13 patients and did not predict for lack of response to IFN. IFN is effective in suppressing the Ph clone in some patients who relapse with CML after allogeneic BMT and controls the blood counts in the majority.