In order to study the effect of oxymetholone therapy in advanced myelofibrosis, 11 patients (4 females, 7 males) were given, 3--5 mg per kg body weight, long-term oxymetholone treatment in a prospective multicenter study. Five cases had previously had a diagnosis of polycythemia vera. All patients had anemia initially, 4 leukocytopenia and 10 thrombocytopenia in addition. Hepato-splenomegaly was present in all cases but in varying degree. Five patients required regular blood transfusions before treatment. In 9 of the 15 courses given, there was normalization of the peripheral blood or substantial improvement (better than 3 g hemoglobin/dl or 50 X 10(9) platelets/1) after androgens. Significant effects were noted both on hemoglobin values and platelet counts. The need for blood transfusions ceased completely in all 5 cases. When oxymetholone treatment was reduced or interrupted 4 patients relapsed; 2 of them responded to a renewed course. The red cell counts returned to previous polycythemic values in one patient and another died from acute leukemia. The results of this study suggest that androgens might be of value in advanced cases of myelofibrosis with transfusion-requiring anemia or severe thrombocytopenia.