Results of a two-year, multicenter clinical experience with the On-X Valve heart valve prosthesis are presented. Between September 1996 and September 1999, 301 On-X valves were implanted at 11 European centers under a standardized protocol. Isolated aortic valve replacement (AVR) was performed in 184 patients, and isolated mitral valve replacement (MVR) in 117. Mean follow up was 24 months in all patients; total follow up was 405.6 patient-years (pt-yr) for AVR, and 219.5 pt-yr for MVR. Actuarial freedom from all valve-related events at two years was 88.7% for AVR and 88.1% for MVR. NYHA class was improved in 75.8% of AVR patients and 70.6% of MVR patients at two years after surgery. These early results indicate that the On-X valve provides satisfactory clinical outcome in the immediate postoperative period, and show that the valve is safe and effective.