 As a result of the presence of large vessel thrombotic complications, as well as the biochemical evidence of ongoing coagulation activation, sickle cell disease SCD is often referred to as a hypercoagulable state. However, the contribution of coagulation activation to the pathogenesis of SCD remains uncertain. While the majority of clinical studies using anticoagulants have shown no convincing benefit in the prevention or treatment of acute pain episodes, most of these studies were small and poorly controlled. Furthermore, because the acute pain episode appears to result from the occlusion of postcapillary venules by the interaction of red blood cells and other cellular elements with the vascular endothelium and subendothelial matrix proteins, it may not be the ideal clinical endpoint for assessing the effect of anticoagulation in SCD patients. Pulmonary hypertension PHT, a common complication associated with significant morbidity and mortality, and with histopathologic findings of in situ thrombosis involving pulmonary vessels, represents a clinical endpoint that is likely due, at least in part, to increased thrombin generation, and may therefore be used to evaluate the contribution of coagulation activation to the pathophysiology of SCD. Twenty patients with sickle cell anemia HbSS or sickle beta zero thalassemia Sickle beta zero thalassemia and mild PHT who meet the eligibility requirements will be enrolled, 10 patients to receive anticoagulation with warfarin and 10 to receive placebo rfor 12 months of treatment. 