Though thromboembolic complications are reportedly minimal in patients with Björk-Shiley aortic valve prostheses, massive thrombosis remains a major and often fatal problem. The authors observed massive thrombosis in 3 patients, only one of whom survived. In all three instances, the disk of the prosthesis was relatively radiolucent. With valves manufactured after late 1975, which contain a tantalum foil hoop, cineradiography is the most rapid and accurate method of showing the disk. A simple method of calculating the valve opening angle from the cineradiograph is described. It is suggested that serial cineradiographs might be useful in detecting lesser degrees of thrombosis and partial immobilization of the disk.