Almost exclusive use of Carpentier-Edwards porcine bioprostheses for cardiac valve replacement in 1194 patients between 1975 and 1987 at the authors' institutions has afforded the opportunity for a more scientific basis for prosthesis selection for subsets of patients, with regard to age and valve-related complications. The present study, performed according to previously established guidelines, investigated the influence of patient age and valve position as determinants of durability of 1315 porcine implants in terms of the following valve-related complications: structural valve deterioration, nonstructural dysfunction, thromboembolism, antithromboembolic therapy-related hemorrhage and prosthetic valve endocarditis. Age groups were as follows: 35 years and younger, 36 to 50 years, 51 to 65 years, and 66 years or older. Thromboembolism was found to be less common in the younger age groups, and significantly less common in patients following aortic compared to mitral valve replacement. The age group assessment for antithromboembolic therapy-related hemorrhage, nonstructural dysfunction and prosthetic valve endocarditis revealed no difference between positions or age groups. Patient age was found to be the major predictor of, and inversely related to, structural valve deterioration. In addition, porcine bioprostheses in the aortic position were shown to have durability superior to that of similar valves in the mitral position. Structural valve deterioration has very little influence on valve-related mortality and residual morbidity. This study thus supports the implantation of porcine bioprostheses in somewhat younger patients than would be advisable for mitral prostheses. The authors now recommend the selective implantation of porcine bioprostheses in the aortic position in patients over 65 years of age, and in the mitral position in patients over 70 years of age.