Thirty three patients with Philadelphia chromosome positive (Ph+) chronic myelogenous leukemia (CML) in relapse after allogenic bone marrow transplantation (BMT) were treated with recombinant alpha-interferon (IFN). Ten patients received IFN for cytogenetic relapse (group I) and 23 (group II) for hematologic relapse. The starting dose of IFN varied from 1.7 to 6 million units/m2/day (median 3 x 10(6) U/m2/day). Among the 10 group I patients, 3 subsequently developed hematologic relapse. Of the other 7, a cytogenetic response was observed in 6 (complete 4, minor 2). Three of these responders are alive in complete cytogenetic remission. Of the 23 group II patients, 3 did not respond to IFN but 20 achieved a complete (CHR) (n = 14) or a partial hematologic response (PHR) (n = 6). Thirteen of the 14 CHR patients subsequently achieved a cytogenetic response (complete 7, minor 6). Seven of the latter 13 patients are still alive in complete cytogenetic remission (CCR). Thus, for the entire group of 33 patients, IFN was followed by CCR in 11 cases (33%); all these patients are still alive and the median follow-up in CCR is now 60.7 months (range 35.3-72.5 months). The BCR-ABL rearrangement was not detected by RT-PCR in 5 of the 10 patients analyzed. Eleven other patients developed either blast crisis or acceleration. The 3-year probability of survival from the start of IFN therapy probability of survival from the start of IFN therapy was 70 +/- 16% (95% CI) and was statistically higher for patients who achieved CCR than for the others.(ABSTRACT TRUNCATED AT 250 WORDS)