Between 1971 and 1975, glutaraldehyde preserved porcine aortic valve xenografts were employed for isolated replacement of the mitral valve (MVR) in 243 patients, replacement of the aortic valve (AVR) in 167 patients, and double valve replacement (AVR and MVR) in 51 patients. Postoperatively, long term anticoagulation was not routinely given. Operative mortality rates for AVR, MVR, and double valve groups were 7.8, 6.0, and 11.8%, respectively; the majority of early postoperative deaths were associated with concomitant coronary artery disease. No death was attributable to xenograft dysfunction. Follow up of all patients was obtained. The total duration of follow up for the MVR groups was 347 patient years, for the AVR group 148 patient years, and for double valve replacement 37 patient years; maximum follow up for these 3 groups was 4.4, 4.0, and 2.4 years, respectively. Actuarial analysis of postoperative survival rates at a common interval of 3 yr showed 78% for MVR patients. 91% for AVR patients, and 80% (projected) for patients with double valve replacement (85, 96, and 91% for operative survivors, respectively). At this same interval 92% of MVR patients, 99% of AVR patients, and 96% (projected) of patients with double valve replacement were free of thromboembolic episodes. Altogether, 12 of the total 512 valves implanted exhibited some evidence of dysfunction during the entire period of follow up evaluation, but in only 2 instances (both mitral) was intrinsic pathological involvement of the xenograft tissue documented. Actuarial analysis of xenograft dysfunction at a common interval of 3 yr after operation showed 95% of MVR patients, 98% of AVR patients, and 97% (projected) of patients with double valve replacement to be free of this complication. These data support the use of glutaraldehyde preserved porcine xenografts as superior bioprostheses that pose a low risk of thromboembolism without anticoagulation. The over all durability of such valves, within the restriction of a maximum current follow up interval of 4.4 years, appears comparable to that of currently available mechanical prostheses and justifies continued clinical use.