Intra-arterial infusion of acetylcholine causes classical endothelium-dependent vasodilatation that is mediated by endothelium-derived relaxing factor(s), the most prominent of which is nitric oxide. The absence of this vasodilatation, or endothelial 'dysfunction', correlates with the earliest stages of atherosclerosis and might be used to identify individuals who could benefit from aggressive intervention strategies and to assess treatment efficacy. Sex steroid hormones (estrogen and testosterone) increase the production and release of endothelium-derived nitric oxide both in men and in women. Therefore, endogenous hormonal status could present a confounding factor in the assessment of endothelial function.