Polycythemia vera was treated in 18 patients in whom the disease had existed for periods ranging from two months to 10 years. Most of the patients had been treated previously by various means, and three were in a state of partial remission (having normal hematocrit readings) at the time of the study. All were treated with busulfan (Myleran) by mouth. Initial dosages ranged from 2 to 10 mg. daily; maintenance dosages ranged from 8 mg. per day to 2 mg. per week. In seven patients this treatment was supplemented with venesection. Complete remission (normal hemograms with absence of signs and symptoms of polycythemia vera) was obtained in 11 patients; incomplete remission (normal hemograms with persistence of one or more signs or symptoms of the disease) was obtained in 6; partial remission occurred in one case. Hematocrit values, which ran as high as 70 in some male patients, returned in every case to normal levels (less than 54% for men). Leukocyte counts were lowered in every case and fell to leukopenic levels (less than 3,500 cells per cubic millimeter) in three. Two patients, whose case histories are given, developed anemia and thrombocytopenia in addition to leukopenia. Close supervision of the patients receiving busulfan therapy is necessary if excessive depression of the bone marrow is to be avoided. The ultimate place of this drug in the treatment of polycythemia vera remains to be determined.