To determine the long-term outcome of biological and mechanical heart valve prostheses, we compared the Biocor, a new generation of porcine bioprosthesis, with the St. Jude Medical mechanical prosthesis. One hundred consecutive patients operated on between 1983 and 1985 with the Biocor in the aortic, mitral, or both positions (without concomitant cardiac procedures) were followed until January 1993, together with 100 matched patients implanted with the St. Jude valve. The mean age was 66 and 67 years, respectively. The follow-up was 100% complete. Valve-related mortality differed significantly, with a freedom rate of 80.7% +/- 9.0% for the St. Jude group and 97.7% +/- 1.6% for the Biocor group. There were few thromboembolic events, with no significant difference between the groups. Anticoagulant-related hemorrhage occurred almost exclusively in the St. Jude group (2.3%/patient-year), and reoperations were required almost exclusively in the Biocor group (1.8%/patient-year). The occurrence of all valve-related complications (mortality plus morbidity) differed markedly, with an actuarial freedom at 10 years of 55.8% +/- 9.8% for St. Jude and 80.0% +/- 4.4% for the Biocor. Quality of life and echocardiographic measurements did not differ significantly between the groups. We conclude from this study that the long-term performance of the Biocor and St. Jude are comparable, even though valve-related complications tended to be fewer in the Biocor group, with a significantly lower incidence of valve-related death.