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Background: Treatment with interferon prolongs survival in chronic myelogenous leukemia. We conducted a clinical trial to assess the efficacy of treatment with a combination of interferon and cytarabine., Methods: Previously untreated patients with chronic myelogenous leukemia were randomly assigned to receive either hydroxyurea (50 mg per kilogram of body weight per day) and interferon alfa-2b (5 million units per square meter of body-surface area per day), or hydroxyurea and interferon in the same dosages plus monthly courses of cytarabine (20 mg per square meter per day, for 10 days). The end points were overall survival, complete hematologic remission at 6 months, and major cytogenetic response (less than 35 percent Philadelphia chromosome-positive cells in the bone marrow) at 12 months., Results: The trial was stopped when a sequential analysis showed a benefit of interferon and cytarabine. A significant improvement in survival was observed in the interferon-cytarabine group (360 patients) as compared with the interferon group (361 patients) (P=0.02; relative risk of death, 0.64; 95 percent confidence interval, 0.44 to 0.93). After three years, the survival rate was 85.7 percent with interferon and cytarabine and 79.1 percent with interferon alone. The rate of hematologic response was higher in the interferon-cytarabine group than in the interferon group (P=0.003). Major cytogenetic responses were observed 12 months after randomization in 126 of 311 patients treated with interferon and cytarabine (41 percent) and in 75 of 314 patients treated with interferon only (24 percent, P<0.001)., Conclusions: The combination of interferon and cytarabine, as compared with interferon alone, increases the rate of major cytogenetic response and prolongs survival in patients with the chronic phase of chronic myelogenous leukemia.