Masses of crystals, which were largely composed of cholesteryl esters, were found in a porcine aortic valvular bioprosthesis removed eight years after implantation in the mitral position in a patient with rheumatic mitral valvular stenosis. Histologic sections of grossly raised and nonraised yellow lesions in the three cusps of this bioprosthesis revealed large clefts, which on frozen section contained lipid-positive, birefringent crystals. These crystals gave a positive reaction with the Schultz test for cholesterol. Biochemical analyses of isolated nodules revealed a cholesterol content of 40 nmole/mg of wet tissue. Of this cholesterol, 88 percent was esterified, and the remaining 12 percent was free cholesterol. These cholesterol deposits are most likely derived from blood lipids; however, they were not related to hyperlipidemia, since the patient had normal blood levels of cholesterol and triglycerides.