Adherence of sickle erythrocytes to endothelium in venules is thought to initiate or propagate vaso-occlusive episodes. Because of blood shear forces with normal microvascular flow, adherence in post-capillary venules requires binding via high-affinity receptor-mediated pathways. Microvascular flow in sickle patients is episodic, even in asymptomatic patients, so adherence may also occur at low shear not requiring high-affinity binding. Sickle cell binding to endothelium was quantified under flow or static incubation with unusually large vWF, thrombospondin, α4β1/VCAM-1 or α4β1/fibronectin (FN). Adherence under flow at 0.5 dyne/cm2 shear stress leads to the greatest number of adherent sickle cells. Adherence under flow at 1.0 dyne/cm2 leads to the strongest adherence. Static incubation conditions promote weak adherence of low numbers of sickle cells to endothelium. Following attachment at 1.0 dyne/cm2, adherence strength was 2.5 ± 0.1 or 2.6 ± 0.2 dynes/cm2 for α4β1/VCAM-1 or α4β1/FN pathways, a level 50% greater than adherence strength mediated by thrombospondin or ULvWF (1.7 ± 0.08 or 1.6 ± 0.07 dynes/cm2, respectively). Sickle cell adhesion promoted by simultaneous activation of α4β1/VCAM-1 and α4β1/FN pathways is the strongest at 6.2 ± 0.2 dynes/cm2 and adherent red cells resist detachment shear stresses up to 10 dynes/cm2. These data demonstrate that sickle cell adhesion to endothelium is regulated both by receptor/ligand affinity and flow conditions. Thus, both microvascular flow conditions and receptor-ligand interactions may regulate sickle cell adherence in vivo. © 2003 Wiley-Liss, Inc.