One hundred fifty four patients underwent isolated mitral valve replacement with bioprosthetic heart valve at Hyogo Medical College Hospital from November 1973 to December 1998. A porcine bioprosthetic valve was replaced in 82 patients (Hancock 43, Carpentier-Edwards 26, Hancock II 13) and pericardial bioprosthetic valve in 72 patients (Ionescue-Shiley 39, Carpentier-Edwards 33) with a mean follow-up of 1,410 patients-years. Their thromboembolism rates were also analyzed in linear and actuarial term over the 15-year period. The incidence of thromboembolism rate was 2.5%/pt.yr. Thromboembolic free rates for patients with anticoagulant therapy were significantly decreased for patients without therapy. Thromboembolic free rates for patients with atrial fibliration were also were significantly decreased for patients with sinus rhythm because the patients with sinus rhythm were not on anticoagulant therapy. In conclusion, it is necessary for the all patients to be on anticoagulant therapy after mitral valve replacement with bioprosthetic valves, even though patients with sinus rhythm.