We report a case of a man with erythrocytosis secondary to hydronephrosis in a right horseshoe kidney. The patient had an advanced hydronephrosis because of ureteropelvic junction obstruction occupying the right hemiabdomen. Following nephrectomy of the right kidney erythrocyte count returned to normal values. Secondary polyglobulia may be due to increased erythropoietine values and has been related in some patients with renal diseases such as hydronephrosis, but very rarely in association with hydronephrotic horseshoe kidney.