Eleven of 14 patients with primary myelofibrosis were given a therapeutic trial with 250 mg of pyridoxine hydrochloride daily because of refractory anemia. The effect on the hemoglobin level and the hematocrit value was studied and compared to that in a group of untreated patients with the same degree of anemia. Six of 11 treated patients responded within three months with a rise in the hemoglobin level (at least 3 g/100 ml) and/or an increase in the hematocrit value (at least 10 per cent), and transfusions were no longer required. Deliberate discontinuation of pyridoxine treatment in one responding patient was followed by a relapse of the anemia; resumption of therapy once again induced an erythropoietic response. Spontaneous remissions of anemia were not observed in the untreated group. It is concluded that a trial with pyridoxine is warranted in patients with myelofibrosis and refractory anemia.