A case of renal cell carcinoma with erythrocytosis is reported. A 58-year-old male had a right renal tumor with retroperitoneal lymph node and lung metastases. Preoperative laboratory data revealed a red blood cell count of 6.31 million/mm3, a hemoglobin concentration of 17.2 g/dl, a hematocrit of 52.1, and an erythropoietin (EPO) level of 47.3 mU/ml, which was determined by RIA method (normal range: 8-30). The patient underwent radical nephrectomy and retroperitoneal lymph node dissection without any blood transfusion because blood loss was only 300 g. The removed kidney weighed 400 g. Pathological examination showed a renal cell carcinoma not invading the fibrous capsule with lymph node metastasis. The EPO activities of extracts from the normal kidney and cancer tissues were less than 18 and 50.8 mU/g, respectively. At present, 3 months postoperatively, he receives daily UFT and HLBI every other day for the lung disease left, which does not change in size, and all laboratory data mentioned above have returned to normal ranges. These results enabled us to diagnose the present case as EPO-producing renal cell carcinoma.