The Hancock Modified Orifice valve is a specially fabricated, trileaflet porcine valve, which has been in clinical use at the Brigham and Women's Hospital since 1976. Three hundred seventy-nine aortic valve replacements with this device were implanted from 1976 to 1985; 305 were isolated aortic valve replacements and 74 were in association with a mitral valve bioprosthesis. The 305 patients undergoing isolated aortic valve replacement, a Food and Drug Administration patient group, are the subject of this article. The mean valve size was 22.5 mm, with 115 21-mm valves used. The mean functional preoperative class was 3.2; 99 of 305 patients (32.5%) underwent concomitant coronary artery bypass grafting. There were six operative deaths (2%). Patients were followed from 6-14 years (mean 7.4 years), and analysis was completed in early 1991. The 10-year actuarial calculation of survival freedom from reoperation, thromboembolism, structural valve degeneration, perivalvular leak, infectious valve degeneration, and total morbidity and mortality are presented.