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Interferon-alpha (IFN-α) is a therapy of unquestionable efficacy in chronic myeloid leukemia (CML) patients. The best dose of IFN-α in the treatment of CML still remains controversial. Our primary objective was to compare cytogenetic responses in patients treated with intermediate versus high doses of IFN-α. A multicenter randomized controlled trial was conducted involving 109 patients with untreated CML in chronic phase from 26 Spanish hospitals. Patients were assigned to receive either an intermediate (2.5 MU/m2 per day) or high (5 MU/m2 per day) target dose of IFN-α. Hydroxyurea was allowed in both groups. In total, 108 patients were analyzed, 53 in the intermediate- and 55 in the high-dose group. Median follow-up was 47.5 months. The dose of IFN-α actually given was lower in the intermediate-dose group (3.83 MU/day) than in the high-dose group (6.6 MU/day) (p<0.001). The rate of complete cytogenetic response was 24.5% in the intermediate- and 12.7% in the high-dose group (NS). A partial cytogenetic response was obtained in 7.5% and 10.9%, respectively. Cox analysis did not reveal any influence of the randomization arm on cytogenetic response rate. Ten patients in each group discontinued IFN-α because of toxicity. Albeit not our primary objective, no differences were found in terms of survival or transformation rate between both groups. Median survival was 73 months; 64% of patients remained free of transformation at 5 years. In terms of cytogenetic response, intermediate doses of IFN-α are as effective as high doses in the treatment of CML. [ABSTRACT FROM AUTHOR]