During the past two years 8 patients were seen with thrombosis of their Bjork-Shiley aortic valves. Six patients were from our series, an incidence of 4% and 2 patients had their original valve implanted at another institution. All patients had substantial problems with anticoagulation therapy. Three died prior to operation. Early detection and emergency surgical intervention is mandatory for survival. At operation debridement of all thrombotic material from the valve provided satisfactory immediate hemodynamic improvements as well as freedom from complications for up to two years. Therapeutic levels of anticoagulation with warfarin are the only apparent protection from thrombosis of the Bjork-Shiley aortic valve.