We report a rare case of hydronephrosis secondary to ureteropelvic stenosis caused by a branch of the left renal vein in horseshoe kidney. We were unable to determine the etiology of hydronephrosis of the horseshoe kidney by means of radiological images. When radiological images fail to show any etiology of hydronephrosis in horseshoe kidney, the possibility of compression of the ureteropelvic junction by a branch of the renal vein should be borne in mind, and re-assessment of the venous phase of angiography and evaluation by coronal or sagittal images of MRI is recommended.