The therapeutic efficacy and toxicity of alpha-interferon (alpha-IFN) (Roferon, Hoffmann-La Roche, Inc., Nutley, NJ) were determined in 15 children (age range, 6 to 20 years) with Philadelphia chromosome-positive chronic myelocytic leukemia (Ph+ CML). All patients had received cytoreductive therapy with either hydroxyurea (n = 13) or busulfan (n = 1) or both (n = 1) for 6 weeks to 46 months (median, 7 months) before beginning alpha-IFN therapy at a dose of 5 x 10(6) U/m2/d intramuscularly. This dose was escalated to 10 x 10(6) U/m2/d if leukemia was inadequately controlled. Ten children had a hematologic response, with nine showing a reduction in the percentage of Ph+ marrow cells, including four who had no detectable Ph+ cells in marrow samples collected 48 to 204 weeks after the initiation of therapy. Two of 15 patients remain free of Ph+ cells. Therapy was discontinued before week 104 in ten patients because of the following: (1) early hematologic responses without a decrease in Ph+ cells (three patients); (2) early resistant disease (one patient); (3) blast crisis (one patient); (4) progressive disease (two patients); (5) seizure attributed to high-dose alpha-IFN (one patient); or (6) an inadequate trial of alpha-IFN caused by aseptic necrosis or poor compliance (two patients). The most common side effects were mild and have included fever, malaise, headache, myalgias, and pain at the injection site. Adverse events causing interruption of therapy were seizures, aseptic necrosis, and myelofibrosis. alpha-IFN stabilizes the chronic phase of Ph+ CML in some children, is adequately tolerated when administered at a dose of 2.5 to 5 x 10(6) U/m2/d intramuscularly, and results in a significant decrease in the proportion of Ph+ metaphases in some patients. alpha-IFN in combination with an effective cytoreductive agent or agents appears worthy of further clinical testing in this disease.