A case of quadricuspid aortic valve is described. The aortic valve consisted of 3 equal-sized and 1 smaller cusps, and a supernumerary cusp located between the right and noncoronary cusps. A right coronary ostium was close to the accessory commissure, and in a lower position. Three fenestrations were found at the supernumerary commissure. Aortic valve replacement was performed successfully with a St. Jude Medical prosthetic valve. On histological examination, the resected cusps showed fibrotic thickening with calcification and no sign of previous inflammatory disease. Quadricuspid aortic valve must be considered a malformation capable of leading to severe valve failure in later life.