Massive thrombosis of a Björk-Shiley aortic valve prosthesis occurs with significant frequency when adequate anticoagulation has not been attained. The converse is also true: This complication is extremely rare in patients receiving anticoagulant therapy. Therefore, we recommend anticoagulants for all patients with Björk-Shiley aortic valve prostheses. Once a diagnosis of a thrombosed prothesis is made, however, immediate operation is indicated. Declotting of the valve without removal of the disc is adequate treatment. After thrombectomy, it is extremely important to evaluate the entire prosthesis critically, with particular attention to the area of the hinge and the occluder. If any wear is observed, the entire prosthesis should be replaced. Excellent long-term results can be expected if the patient is maintained on adequate anticoagulation postoperatively.