Prosthesis choice for aortic and mitral valve replacements in patients aged 61-70 years is difficult. We evaluated prostheses in age groups 61-65 and 66-70 years. Freedom from major thromboembolism and hemorrhage was greater for bioprostheses than mechanical prostheses in both age groups after aortic valve replacement, but only in the younger age group after mitral valve replacement. Freedom from valve-related re-operation was greater after mitral valve replacement with mechanical prostheses in both age groups, but no difference after aortic valve replacement. Valve type was predictive of major thromboembolism and hemorrhage, except in older patients undergoing mitral valve replacement. Bioprostheses are favored for aortic valve replacement in both age groups, but the risk of re-operation with a bioprosthesis in the mitral position in patients aged 61-65 years favors a mechanical prosthesis. Prosthesis choice is less definite in those aged 66-70 years.