In 12 patients having myelofibrosis with myeloid metaplasia (MMM), recombinant- alpha interferon (r-alpha INF) was given for 16 weeks at an initial dose of 3 x 10(6) U/day as a cytoreductive agent. At the end of the 16th wk, Hb showed minor changes; WBC were reduced from 43 x 10(9)/l, range 6.4-69.4, to 16 x 10(9)/l, range 5-39 (p = 0.05); platelets decreased from 845 x 10(9)/l, range 215-1748, to 370 x 10(9)/l, range 96-730 (p = 0.005). 2 cases responded at the starting dose, while the effective dose was 5 x 10(6) U/d in the others. Minor changes in spleen size were noted, while no significant changes in bone marrow fibrosis occurred. After induction therapy, 3 patients were allocated to maintenance therapy (from 10 up to 34 months). To maintain platelet count lower than 500 x 10(9)/l, the required r-alpha-INF dose was constantly 10 MU/wk, while the same result was not achieved in 1 case with hydroxyurea, 1 g/die. The association with hydroxyurea, 500 mg/die, allowed reduction of the r-alpha INF dose to 6 MU/die in 1 other case.