Cardiac bioprosthetic valve thrombosis is frequently found on pathologic examination, but preoperative diagnosis is rarely performed. Four hundred six patients with mitral porcine xenograft bioprostheses were examined by transthoracic echocardiography. Transesophageal echocardiography (TEE) was performed in 161 of the patients, with clinical or echocardiographic criteria of prosthetic malfunction. Fairly homogeneous and echodense masses, attached to the ventricular surface of the mitral bioprosthetic cusps, were detected by TEE in 15 patients. Only 10 patients, in whom diagnosis of bioprosthetic thrombosis was confirmed, are included in this study. After TEE, two patients underwent prosthetic replacement and eight patients received anticoagulants. A new TEE was performed 85.6 +/- 29.8 days after anticoagulation in these eight patients. Clinical follow-up was continued for 13.6 +/- 8.6 months, and one additional patient underwent surgery during the follow-up. Pathologic examination of removed grafts (three cases) identified these masses as being thrombotic tissue. TEE examination after therapeutic anticoagulation demonstrated complete disappearance of the echogenic masses on bioprosthetic cusps and normal mobility of all leaflets in six cases. In the other two cases, cusp masses were notably reduced, but partially restrictive mobility of affected leaflets persisted, suggesting incomplete resolution of thrombi. Mitral valve prosthetic mean gradient decreased from 11.8 +/- 4.5 to 7.6 +/- 3.7 mm Hg (p < 0.001), and mitral valve area increased from 1.13 +/- 0.3 to 1.72 +/- 0.6 cm2 (p < 0.001). Long-term symptomatic improvement after anticoagulation was obtained in seven patients. Thus this study shows that mitral bioprosthetic thrombosis is a relatively frequent cause of valve dysfunction, TEE is useful for detecting thrombus in relation to mitral bioprosthetic valves, and oral anticoagulation is effective in resolving thrombosis on bioprostheses.