A 44-year-old white male was referred to our hospital for evaluation of asymptomatic polycythemia which had been detected on a routine checkup. On admission, the patient described a mild headache for about 1 month without nausea or vomiting. There was no history of thromboembolism and no pruritus. As a passionate cyclist, he was a nonsmoker and denied using stimulating drugs such as exogenous erythropoietin (EPO). On physical examination the patient seemed in good general health, his blood pressure was 150/100 mmHg, and his pulse rate 60 beats/min. Auscultation of the heart and lungs was unremarkable and no neurologic deficits were found. Laboratory studies showed markedly increased hemoglobin at 20.2 g/dL, a hematocrit of 63%, and slightly elevated uric acid. The white blood and platelet counts were normal. Bone marrow (Fig 1; hematoxylin and eosin, 40) revealed increased, left-shifted erythropoiesis, normal granulopoiesis and megakaryopoiesis, and no evidence of a myeloproliferative disorder. Measurement of the total RBC mass by isotope dilution with cromium-51–labeled red cells demonstrated marked erythrocytosis at 42.4 mL/kg body weight (normal expected value, 27 mL/kg body weight) and reduced plasma volume. The serum EPO level was elevated at 32 U/L (normal, 5 to 25 U/L). Evaluation for secondary erythrocytosis showed no evidence of an EPO-producing tumor on the computed tomography scan of the chest and abdomen, or increased EPO due to hypoxemia. Eventually, because there is a known association of cerebral hemangioblastoma with elevated EPO levels, magnetic resonance imaging (MRI) of the cerebrum was performed. The MRI scan (Fig 2) disclosed a tumor measuring 3 4 cm in diameter in the right cerebellar hemisphere (black arrow) with perifocal edema and midline shift (white arrow), resulting in obstruction of the fourth ventricle (thin arrow). Radiologic findings were consistent with the criteria of a solid cerebellar hemangioblastoma. With the clinical diagnosis of an EPO-producing hemangioblastoma in the posterior fossa, the patient was referred to neurosurgery. The tumor could be resected completely and the patient had no neurologic deficit postoperatively. Histology (Fig 3; 40) confirmed the diagnosis, showing the characteristic findings of a capillary hemangioblastoma WHO grade 1 (A, hematox-