The limited effects of current treatments of primary myelofibrosis (PM) led us to prospectively evaluate recombinant interferon-α (rIFNα) in "early" PM patients with residual hematopoiesis and only grade 1 or 2 myelofibrosis. Seventeen patients meeting World Health Organization PM diagnostic criteria received either rIFNα-2b 500 000 to 3 million units 3 times weekly, or pegylated rIFNα-2a 45 or 90 μg weekly. International Working Group for Myelofibrosis Research and Treatment criteria for prognosis and response were used. Eleven patients were women and 6 were men. Their median age at diagnosis was 57 years. Eleven patients were low risk and 6 were intermediate-1 risk. Two achieved complete remission, 7 partial, 1 clinical improvement, 4 stable disease, and 3 had progressive disease. Thus, more than 80% derived clinical benefit or stability. Improvement in marrow morphology occurred in 4. Toxicity was acceptable. These results, with documented marrow reversion because of interferon treatment, warrant expanded evaluation.