Seven cases of massive thrombosis of an aortic Bjork-Shiley prosthesis were encountered among 433 valvular implants. Four patients died before treatment could be instituted, and three underwent repeat surgery successfully. Anticoagulation therapy was probably well maintained in four, and poorly maintained in three patients. These cases illustrated the need for a high index of suspicion and prompt recognition of this complication in patients with Bjork-Shiley prostheses. The diagnosis of massive thrombosis should be suspected with the rapid onset of (1) signs of congestive heart failure, (2) absence or attenuation of valvular clicks, (3) aortic regurgitation, or (4) hemolytic anemia. Salvage of these patients requires emergency replacement of the thrombosed prosthesis or removal of the thrombus.