The thromboembolic rate of 768 patients who were treated only with aspirin after mitral valve replacement or mitral plus aortic valve replacement with porcine bioprostheses was evaluated. We analyzed the thromboembolic rate for the whole series and for subgroups of patients categorized by atrial fibrillation, giant left atrium, left atrial thrombosis, and dosage of aspirin (1 gm daily or 0.5 gm every 48 hours). The total embolic rate was 1.4% (11/768). No patient in sinus rhythm had an embolic event. The embolic rate for patients in atrial fibrillation was 1.9% (11/583). There were no embolic events in 31 patients with a giant atrium. An embolic event occurred in 1 of 42 patients with atrial thrombosis (2.4%). Patients treated with 1 gm of aspirin daily had a 3% embolic rate (9/295) while the incidence was 0.4% (2/473) in those treated with 0.5 gm every 48 hours (p less than 0.01). Administration of aspirin after mitral valve replacement with a bioprosthesis is a very effective treatment for prevention of thromboembolism. In our experience, this treatment provides protection equal to or better than that offered by oral anticoagulants for patients in atrial fibrillation as well as for patients with a giant atrium or atrial thrombosis at operation. The dosage and timing of aspirin administration may markedly affect the result of this type of treatment. Oral anticoagulation with coumarin derivatives may not be appropriate after mitral valve replacement with a bioprosthesis, and platelet antiaggregates should be used for this purpose in the future.