The presence of arterial aneurysm--an abnormal dilatation of an arterial segment due to various causes--might lead to compression effects and to various symptoms. Finding an etiology for unilateral hydronephrosis represents in some cases a real challenge for the clinician and targeted investigations must be ordered for the diagnostic approach. The abdominal ultrasound examination is one of the first imaging modalities but the abdominal computed tomography is helpful for definitive conclusions. We present a rare case of unilateral hydronephrosis due to a giant left iliac artery aneurysm in a 77-year-old male with history of peripheral artery disease.