In a 5-year period 52 consecutive patients underwent aortic valve replacement with the Lillehei-Kaster pivoting disc prosthesis. Forty-four patients surviving more than one year after the operation were followed up to determine the incidence of thromboembolism and hemorrhage. In group I 24 patients with a mean observation time of 24.9 months (range 2-60) had anticoagulants discontinued 6 or 12 months postoperatively and showed a thromboembolic rate of 8.0 per 100 patient-years and a hemorrhagic rate of 2.0 per 100 patient-years. Group II contained 23 patients with a mean observation time of 16.3 months (range 2-44) and showed a thromboembolic rate of 6.4 per 100 patientyears and a hemorrhagic rate of 9.6 per 100 patient-years. Patients, who suffered a thromboembolic episode, showed a mean postoperative valve gradient of 37 mmHg, significantly higher than the mean postoperative valve gradient of 17 mmHg for the rest of the patients. The thromboembolic risk showed no clear-cut tendency to decline with time. No thrombotic encapsulation of the prosthesis was found. No clear correlation between hemorrhages and prothrombinproconvertin time was found. The actuarial 5-year survival rate for patients leaving hospital was 77.3 %.