Records from 99 patients who received the Delrin-disk model (Group D) of the Björk-Shiley (BS) prosthesis between December, 1971 and November, 1974 were reviewed and compared to those from 248 patients who received the Spherical-disk model BS between January, 1975 and June, 1981 (Group S). Complete follow-up was obtained in 98.8% of the patients. The follow-up was over 1302.3 patient-years (mean 13.2 years per patient) in Group D and 2967.5 patient-years (mean 12.0 years per patient) in Group S. Early mortality was not significantly different (Group D: 18.2%, Group S: 10.5%). Of 15 late deaths in Group D, 12 (60%) were valve related and of 58 late deaths in Group S, 34 (58.6%) were valve related. The survival proportion based on Kaplan-Meier analysis, and which included early deaths, was equivalent in both groups (Group D: 65.7% at 21 years, Group S: 64.8% at 18 years). The freedom from reoperation was 74.3% at 21 years for Group D and 95% at 18 years for Group S (p < 0.005). The incidence of prosthetic valve dysfunction in Group D was significantly higher than that in Group S during 15 years after insertion of the valve. The freedoms from thromboembolism and valve-related events were not significant in both groups. Gross examination of the explanted Delrin disk found wear indentations and excessive clearance between the disk and the prosthetic ring. Therefore, disk wear was the primary cause of prosthetic valve dysfunction for the Delrin model. These results suggest that patients with the Delrin-disk model of the Björk-Shiley prosthesis should receive close follow-up for structural integrity of the valve. Further, reoperation might be needed due to significant valve dysfunction in this group of patients.