Twenty-seven patients with p. vera whose proliferative disease required the suppression of bone marrow function were treated with melphalan for periods of 20 to 72 months. The response was rated as good to excellent at three months in 24 of the 27 patients. At the end of one year 14 of the 27 patients had no evidence of disease. In eight of the remaining 13 the only persistent abnormality was a palpable spleen. Side reactions and adverse hematologic effects were infrequent. To date one patient has developed refractory anemia and evidence of myelofibrosis. Four patients have developed acute leukemia but none of these had been treated with melphalan alone. The results to date are sufficiently good to establish melphalan as one of the most effective agents in the control of p. vera.