Complex pertubations of hemostasis occur in sickle cell disease (SCD). Although the procoagulant property of sickle erythrocytes in vitro is tied to exposure of phosphatidylserine (PS), no study has directly linked this PS positivity to in vivo thrombin generation. This study was designed to determine if thrombin generation in SCD correlates with erythrocyte PS, or whether platelets play a significant role. PS was quantified on erythrocytes and platelets from 40 patients with SCD (SS genotype = 25; SC genotype = 15) and 11 controls. Markers of thrombin generation (prothrombin fragment F1.2; thrombin-antithrombin or TAT complexes) and fibrin dissolution (D-dimer; plasmin-antiplasmin or PAP complexes) were also evaluated. Thrombin generation and activation of fibrinolysis occurred with elevations in F1.2, TAT, and D-dimer. Although numbers of both PS-positive erythrocytes and platelets were elevated, there was no correlation between PS-positive platelets and any hemostatic markers. In contrast, correlations were noted between PS-positive erythrocytes and F1.2 (P <.0002), D-dimer (P <.000002), and PAP (P <.01). Correlations between F1.2 and D-dimer (P <.0001) demonstrated that fibrinolysis was secondary to thrombin generation. In patients with the SC genotype, abnormalities in coagulation, although present, were of a lesser magnitude than in SS disease. This study suggests that the sickle erythrocyte is the cell responsible for the thrombophilic state in SCD because associations between erythrocyte PS and thrombin generation were observed. No such relationship with platelet PS was noted. The use of erythrocyte PS as a surrogate marker in trials testing new therapeutic modalities may provide insights into the vascular complications of SCD.