Three cases of prosthetic thrombosis with the SJM mechanical bileaflet pyrolitic valve in the aortic position occurred in a series of 355 patients operated on over a four-year period (October 1978-September 1982). Symptoms and signs of inadequate left ventricular flow (with various degrees of aortic regurgitation) were marked and developed rapidly. The patient is usually aware of muffling of the valve sounds. Auscultation is typical (muffled or abolished valve sounds; appearance of a systolo-diastolic murmur). Echocardiogram and carotidogram are suggestive. Adequately oriented fluoroscopy is diagnostic. Emergency prosthetic replacement is life-saving. Long term, well controlled, anticoagulation with coumadin is advocated.