Starting in February 1978 52 St. Jude Medical Prostheses have been implanted in 47 patients (age 13--73 years) in aortic, mitral and tricuspid positions. Perioperative death occurred in 3 cases. Post mortem examination of these patients showed unimpaired valvular function. There were no other complications in an accumulated total valve time of approximately 300 months. Prior to surgery and post-operatively, several hemolytic parameters were analyzed. There was only a mild elevation in Serum LDH (285 +/- 10 iV/L). Fifteen patients underwent recatheterization 3 months postoperatively. The mitral valves did not reveal an enddiastolic gradient at rest, while the aortic valves had a peak to peak gradient of 8 +/- 2.1 mmHg. A standardized stress consisting of an infusion of isoproterenol was used. The results were compared to results obtained in patients with Björk-Shiley prostheses under identical conditions and did not show a significant difference. The preliminary hemodynamic findings correlate with preceding in vitro results and preclinical data, and the first clinical experience is promising.