A 75-year-old man with quadricuspid aortic valve regurgitation affected by bacterial endocarditis is reported. The aortic valve consisted of 4 equal-sized cusps (type a) and a supernumerary cusp located between the right and noncoronary cusps. A right coronary ostium was close to the accessory commissure, but there was no displacement. A few small fenestrations were found at the 4 commissures and a large perforation resulting from endocarditis was observed in the noncoronary cusp. 2-D echocardiogram and angiogram suggested these findings, and they were confirmed at surgery. Successful aortic valve replacement was achieved.