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Brachial artery reactivity following compression is used as a surrogate marker of endothelial function. Brachial diameter increases exceeding 10% indicate normal endothelial function. We investigated if basal brachial artery tone influenced brachial artery reactivity. One hundred never smoking, healthy, normotensive, normolipidaemic subjects (41+9 yrs, 71F, 29M) underwent brachial artery assessments. Basal, Reactive (endothelial dependant diameter change following 4minutes of cuff compression) and Post GTN (endothelial independent dilatation) diameters were recorded. Tone was calculated as the percent difference in Post GTN and Basal brachial diameters. Basal diameter 3.6+0.05mm, Reactive diameter 4.0+0.06mm, Post GTN Diameter 4.4+0.1mm There was a significant correlation between Brachial artery reactivity and basal brachial artery tone (All r=0.60, p<0.0001, Women r=0.57, p<0.0001, Men r=0.68, p<0.0001). These data indicate that basal brachial tone has a strong association with the degree of brachial artery reactivity in men and women and consideration should be given to the influence of tone on percent reactivity when measuring endothelial function by this methodology. A Reactivity/Tone ratio should be considered once new cut-off points are determined.