Thromboembolism is a serious problem which is incompletely controlled by currently available anticoagulant drugs. The objectives of this study are to define the role of platelets in recurrent venous thrombosis and in the embolism associated with rheumatic mitra valve disease, by measuring platelet survival time (51 Chromium)and determining the effects of platelet inhibitor therapy with Sulfinpyrazone. Recent studies suggest that some patients with recurrent venous thrombosis have shortened platelet survival and that thrombosis and platelet survival are improved with sulfinpyrazone. This study will answer the following questions: (1) What is the frequency of abnormal platelet and fibrinogen survival in recurrent venous thrombosis? (2) What is the relationship between survival of fibrinogen and platelets in these patients? (3) What is the effect of blocking fibrinogen (Heparin) on platelet survival and the effect of blocking platelets (Sulfinpyrazone) on fibrinogen survival? In valvular heart disease and following valve replacement, studies suggest that platelet survival correlates with thromboembolism. Platelet survival will be measured serially in patients with mitral stenosis to answer the following questions: (1) Does abnormal platelet survival develop with progression of mitral stenosis? (2) Does platelet survival reliably predict thromboembolism? (3) Will platelet inhibitor therapy reduce thromboembolism in mitral stenosis? A clinical trial will be undertaken which will compare the course of mitral stenosis relative to thromboembolism in patients with normal or shortened platelet survival time who are either untreated or on sulfinpyrazone therapy.