From July 84 to June 88, 100 patients underwent an isolated aortic valve replacement by a Monostrut-Björk-Shiley prosthesis. Fifty-one had calcified aortic stenosis, 24% aortic insufficiency, 25% mixed aortic lesions. Pre-operatively, their mean age was 57 years, 68% were male, 46% were in NYHA class II or IV, 43% had angina, all were in sinus rhythm. Operative mortality was 4%. All the survivors were followed-up for a mean period of 22 months (6 to 58) with a cumulative follow-up of 183 patients-years. All patients were placed on a long-term regimen of anticoagulant therapy. Two late deaths occurred (1 myocardial infarction, 1 cerebral hemorrhage). The 4 years survival rate was 94%. Postoperative functional results were excellent. Nearly all patients were asymptomatic. Concerning valve related complications: the 4 years actuarial rate of patients free from thromboembolism, hemorrhage, valve thrombosis, periprosthetic leakage and endocarditis were respectively 97%, 97%, 100%, 100% and 100%. No patient were reoperated on. Valvular function was evaluated by mean transprothetic gradient on echo-doppler (61 cases) and by the calculation of the valvular area with transseptal catheterization (21 cases). For small sizes prosthesis (annulus diameter of 19 or 21 mm), medium size prosthesis (23 or 25 mm), large size prosthesis (27 to 29 mm), mean transprothetic gradient were respectively 16, 10 and 6.9 mmHg and valvular aortic area were respectively 1.5, 1.9 and 2.4 cm2. Mid term results of the Monostrut-Björk-Shiley prosthesis on aortic position are good with a low rate of valve related complications and good hemodynamic results, even with the small valve size.