A 48-year-old man had angina pectoris and symptoms of heart failure. Cardiac catheterization showed severe aortic stenosis and regurgitation, and he underwent aortic valve replacement in 1976 with a 23-mm Carpentier-Edwards aortic porcine heterograft. Initially there was symptomatic improvement, but one year later the patient had the onset of dyspnea and exertional chest pain that became progressively more severe over the next three years. Repeat cardiac catheterization was performed 45 months after valve implantation, and a 76 mm Hg peak to peak and a 44 mm Hg mean systolic gradient across the aortic bioprosthesis were recorded. The porcine heterograft was surgically explanted and found to be obstructed by a subvalvular thrombus that appeared to arise from the junction of the aortic and mitral valve annuli.