The effect of treatment with 1,25-dihydroxyvitamin D3 administered at the dose of 1.50-3.00 ug/day for at least 12 months was evaluated in three patients with idiopathic myelofibrosis and in five patients with idiopathic thrombocythemia. This treatment did not cause any significant change in the hematological values or the clinical course of the myeloproliferative diseases in any of the patients. Based on these data, treatment with 1,25-dihydroxyvitamin D3 in non toxic doses seems to be of doubtful benefit in patients with these disorders.