Horseshoe kidney is an inborn renal fusion anomaly. It is frequently associated with renal stones and infections. The diagnosis is made using imaging methods - ultrasound, X-ray of the abdomen and intravenous pyelography, computed tomography (CT), magnetic resonance imaging (MRI) and radionuclide investigations. The diagnosis sometimes is hard, especially when other abnormalities are present, i.e. hydronephrosis, nephrolithiasis, stenosis of the ureteropelvic junction, etc. The authors present a male patient with horseshoe kidney and unilateral hydronephrosis due to obstruction of the ureteropelvic junction and discuss the diagnosis of horseshoe kidney and the diagnostic approach in such patients.