Oxidative stress plays a major pathogenetic role in cardiovascular disease. The C242T variant of the CYBA gene encoding the p22phox subunit of the NAD(P)H oxidase, a major source of superoxide production, has been shown to be associated with coronary artery disease and with vascular superoxide production in human veins ex vivo. Since superoxide degrades nitric oxide (NO), we hypothesized that the C242T variant influences endothelium-dependent vasodilation of the human forearm vasculature in vivo. In the present study, 90 subjects with elevated cholesterol levels were stratified for the C242T polymorphism of the CYBA p22phox gene. Endothelium-dependent and -independent vasodilation were assessed by plethysmographic monitoring of forearm blood flow responses to intra-arterial infusion of acetylcholine and sodium nitroprusside respectively. N(G)-Monomethyl-L-arginine (L-NMMA) was infused to analyse NO-mediated basal vascular tone. Baseline parameters (age, gender, blood pressure, body mass index, cholesterol level) were similar across the genotypes. No differences in forearm blood flow responses to the intra-arterial infusion of acetylcholine, sodium nitroprusside or L-NMMA were found across the CYBA p22phox genotypes. Our sample size of n =90 had a power of >80% (beta=0.20) with a P value of <0.05 (alpha=0.05) to detect a difference greater than 156% in the forearm blood flow response to acetylcholine across genotypes (S.D. 336%; average increase in forearm blood flow=514%). In conclusion, at a power of 80%, our study excludes a major effect of the C242T CYBA p22phox polymorphism on acetylcholine-mediated endothelium-dependent vasodilation and basal NO-mediated vascular tone of the human forearm circulation in subjects with hypercholesterolaemia.