Adherence of erythrocytes to vascular endothelium likely contributes to the pathophysiology of episodic vascular occlusion in patients with sickle cell disease (SCD). In addition, coagulation activation has been reported in sickle patients during complications such as pain episodes. To test the hypothesis that platelet activation contributes to sickle erythrocyte binding, we investigated whether factors released from activated sickle platelets promote adherence of sickle erythrocytes to human microvascular endothelial cells (MEC) under flow conditions. Activated sickle platelet supernatant (ASPS) promoted high levels of sickle erythrocyte adherence to MEC (55.4 ± 3.9 erythrocytes/mm2) but only moderate adherence of normal erythrocytes to MEC (14.1 ± 0.7 erythrocytes/mm2). When MEC were incu-bated with an antibody (OKM5) against CD36 (a thrombospondin [TSP] receptor), platelet supernatant mediated sickle erythrocyte adherence was inhibited 86%, suggesting that TSP participated in the adherence. To further define the role of TSP in adherence, additional studies using purified TSP were performed. At a concentration of 0.2 μg/ mL TSP in serum-free media (SFM), sickle erythrocyte adherence to MEC was 33.9 ± 2.7 erythrocytes/mm2 and sixfold greater than either sickle erythrocyte adherence in the absence of TSP or normal erythrocyte adherence in the presence of TSP. Doubling the concentration of TSP to 0.4 μg/mL proportionally increased adherence of sickle erythrocytes. Incubation of MEC with OKM5 or anti-αν monoclonal antibodies inhibited TSP-mediated sickle erythrocyte adherence more than 95%. These data suggest that activated platelet release factors, including α-granule TSP, which promote receptor-mediated sickle erythrocyte adherence to microvascular endothelium. Such factors released during in vivo platelet activation could contribute to vaso-occlusive complications by promoting erythrocyte adherence and microvascular occlusion.