A 63-year-old man with idiopathic myelofibrosis, diagnosed on Sept. 2004 and treated with hydroxyurea (HU), was admitted to our hospital on March 2005 because of dyspnea and pleural effusion on the right side. Effusion analysis revealed hemothorax due to extramedullary hematopoiesis or bleeding. We performed a thoracoscopic pleural lung biopsy and pleurodesis with minocycline after surgical evacuation. However, bilateral pleural effusion appeared only 2 weeks later, which was thought to be extramedullary hematopoiesis or acute transformation. We adopted a treatment strategy with low dose whole lung irradiation (4.25 Gy/5 fractions) which was followed by complete disappearance of the bilateral pleural effusion. Our present case suggests that low dose whole lung irradiation may be an effective and safe therapy for intractable pleural effusion due to extramedullary hematopoiesis.