Introduction: Despite the intriguing in vitro data implicating vWF in sickle-RBC-endothelial adhesion and in animal models of vasoocclusion, there are few clinical studies characterizing the role of vWF in SCD pathophysiology. Rationale: Plasma vWF is mainly endothelial-derived, upto 95% of newly synthesized vWF being secreted constitutively. Since in vitro studies have shown that hypoxia mediates release of ultra-large vWF multimers stored in endothelial Weibel Palade bodies, we hypothesized that changes in the plasma vWF profile of patients with sickle cell disease (SCD) could potentially be related to their oxygenation status and subsequent clinical events. Patient groups: Following informed consent blood samples were obtained from 20 children with SCD who were enrolled in an investigation of sleep hypoxia. After an awake blood oxgygen saturation by pulse oximetry (SaO2), an overnight recording was performed and a histogram generated showing proportions of time spent at various SaO2s. On the basis of mean sleeping SaO2s, the study population was divided into Group I (Normoxia, sleeping SaO2>94%, n=10) and Group II (Hypoxia, sleeping SaO2 <94%, n=10). Methods: VWF antigen level (vWF antigen) was determined by a latex enhanced immunoassay and Ristocetin Cofactor activity (RCof) was evaluated by platelet aggregometry. VWF multimer analysis was performed on SDS- 1.6% agarose gels and visualized using a polyclonal rabbit anti-human VWF antibody (Dako) followed by chemiluminescent detection using horse radish peroxidase-conjugated anti-rabbit IgG (Amersham). The presence of high molecular weight VWF multimers had to fulfill the following three criteria: (a) vWF multimers larger than the largest vWF multimers present in pooled normal plasma;(b)vWF multimers similar to the largest multimers present in a post-DDAVP infusion plasma sample analyzed on the same gel;(c) assessment by three independent investigators. Student’s t-test was performed to compare the mean values of vWFAg, vWF:RCof, and vWF RCof:Ag ratio of Gps I and II. Results: We observed significant differences between normoxic vs. hypoxic children with SCD as depicted in the table below where results are shown as Mean values(1SD). vWF antigen levels correlated inversely (r2=0.28) and vWF RCof:Ag ratios correlated positively (r2=0.2) with sleeping SaO2s. vWF:Ag* vWF RCof vWF RCof:Ag Ratios** HMW-vWF *p <0.003; **p <0.006 Group I >/94% 120.8(30.9) 116.7 (36.8) 0.96(0.1) 0/7 (performed to date) Group II <94% 192.2 (69.4) 147.3 (71.3) 0.77(0.14) 4/5 (performed to date) Conclusions: Children with SCD and sleep hypoxia showed abnormalities in their vWF profiles when compared to normoxic children with SCD including elevated vWF antigen and qualitatively increased HMW multimers suggestive of hypoxia-mediated endothelial response. In addition, there is an unexpected discordance between these findings and decreased vWF RCof:Ag ratios. Since children with SCD and nocturnal hypoxemia have an increased rate of pain-associated vasoocclusion and CNS complications, our findings of vWF-related perturbations could provide an explanation for hypoxia-induced modulation of the proadhesive and prothrombotic state in these children.