Quadricuspid semilunar valves are uncommon congenital anomalies sometimes associated with severe valvular dysfunction. Two cases of isolated quadricuspid semilunar valve were identified. The first, a heavily calcified quadricuspid aortic valve with four equal-sized cusps, was identified in a 56-year-old man at the time of valve replacement surgery for severe aortic stenosis and regurgitation. The second valve was a quadricuspid pulmonary valve with three equal-sized cusps and one smaller cusp, identified incidentally at autopsy in a 79-year-old woman with a ruptured abdominal aortic aneurysm. Review of the literature found that while quadricuspid pulmonary valves are infrequently associated with serious clinical complications, the majority of quadricuspid aortic valves are associated with clinically significant dysfunction, most commonly aortic insufficiency. Further, contrary to previous thinking, the frequency of aortic valvular dysfunction does not appear to be related to valve morphology.