Secondary polycythemia is a condition for which the red blood cell count is increased due to the physiologic response to hypoxia, autonomous erythropoietin production or deliberate erythropoietin administration. Hydronephrosis and postrenal obstruction have rarely been reported as the causes of secondary polycythemia. We report here on a case of a 30-year- old man with secondary polycythemia that was caused by giant hydronephrosis due to ureteropelvic junction obstruction, and this patient was successfully treated by nephrectomy.