In 30 consecutive splenectomies for myelofibrosis (MF) with cytopenia(s) and/or massive splenomegaly the operative mortality was 6.7% (2 patients). Postoperative complications were observed in 11 (37%) patients. When surgery was undertaken within one year of the diagnosis of MF the postoperative morbidity rate was 13% (2/16), but significantly (p less than 0.01) higher at 64% (9/14) in patients operated on later. A similar significant difference was observed in the amount of intraoperative blood loss. Of the patients undergoing splenectomy for anaemia and/or thrombocytopenia, 79% gained definite benefit from the operation for 2 to 70 months (median 10 months). It is concluded that, although splenectomy probably does not prolong the life of patients with MF, it gives considerable symptomatic relief in cases with massive splenomegaly, and can improve the patient's haematological status and reduce blood transfusion requirements. To avoid the increased risks of postoperative complications surgery is best undertaken as soon as cytopenia is present.