The vessel wall endothelium undoubtedly plays a role in the vascular pathobiology of sickle cell disease. This pilot study tested the feasibility of using an inhibitor of nuclear factor (NF)-κB, a transcription factor, to modify the endothelial activation state of patients with this vascular disease. For a total of 7 separate drug exposure tests, 3 subjects with sickle cell disease took sulfasalazine (given orally at 1 g every 8 hours), and the activation state of their circulating endothelial cells (CECs) was assessed using immunofluorescence microscopy. Companion studies were also performed using sulfasalazine in sickle transgenic mice to verify its effect simultaneously on both CECs and vessel wall endothelium. Both CECs and tissue vessel wall endothelium in sickle mice have an activated phenotype. In these mice sulfasalazine significantlly reduced CEC expression of vascular cell adhesion molecule (VCAM), intracellular adhesion molecule (ICAM), and E-selectin, and it correspondingly reduced expression of these molecules in some tissue vessels. In humans with sickle cell disease, sulfasalazine significantly reduced CEC expression of VCAM, ICAM, and Eselectin, but it did not reduce expression of tissue factor. Addition of a second transcription factor inhibitor, salsalate, did not change this result. This pilot study suggests that endothelial cell activation state can be modified and down-regulated in vivo by sulfasalazine.