We retrospectively analyzed the outcomes of 14 patients with primary myelofibrosis who were treated with splenic irradiation (SI) for symptomatic splenomegaly between January 2000 and December 2012 at 12 hospitals. Median age at the time of SI was 67 years (range 47-76). The median dose of radiation per course was 5 Gy, administered in a median of eight fractions. Spleen size was reduced in 93 % of patients, and persisted for a median of 2.2 months (range 0.1-13.8). Symptom relief occurred in 86 % of patients, and lasted for a median of 2.5 months (range 0.1-16.5). Although SI provided a high rate of palliation for patients with symptomatic splenomegaly, the responses were transient. Significant thrombopenia (<25 x 10(9)/L) occurred in eight patients (57 %), and neutropenia (<0.5 x 10(9)/L) was observed in seven (50 %). Nine patients (64 %) required an increased number of red blood cell transfusions after SI. Five patients (36 %) developed serious infections, with two deaths (14 %), as a result of SI-induced cytopenia. The median survival for all patients after SI was 18.5 months (range 0.1-71.9). The Dynamic International Prognostic Scoring System model effectively distinguished the prognosis after SI between patients in the intermediate-2 and high-risk groups.