Hydroxyurea (HU) is frequently given as treatment for myelofibrosis (MF), but data on its efficacy and tolerability are scarce. The results of HU therapy were evaluated in 40 patients with hyperproliferative manifestations of primary (n = 32), post-polycythemia vera (n = 6), or post-essential thrombocythemia (n = 2) myelofibrosis. Median interval between diagnosis and HU start was 6.2 months (range 0-141.7). Reasons for treatment were constitutional symptoms (55%), symptomatic splenomegaly (45%), thrombocytosis (40%), leukocytosis (28%), pruritus (10%), and bone pain (8%). The starting dose was 500 mg/day, subsequently adjusted to the individual efficacy and tolerability. Response was bone pain 100%, constitutional symptoms 82%, pruritus 50%, splenomegaly 40%, and anemia 12.5%. According to the International Working Group for Myelofibrosis Research and Treatment criteria, clinical improvement was achieved in 16 patients (40%). Median duration of response was 13.2 months (range 3-126.2). Worsening of the anemia or appearance of pancytopenia were observed in 18 patients, requiring administration of erythropoietin-stimulating agents (n = 17) and/or danazol (n = 9). Oral or leg ulcers appeared in five patients and one had gastrointestinal symptoms. HU is an effective and generally well-tolerated therapy for the hyperproliferative manifestations of MF. The accentuation of the anemia often induced by HU is usually manageable with concomitant treatment.