OBJECTIVE The aim of this study was to assess flow-mediated dilatation (FMD) of the brachial artery in patients with acromegaly. METHODS We prospectively evaluated 25 patients with acromegaly (14 females, 11 males; aged 42 +/- 12 years; growth hormone (GH) levels 34 +/- 14 ng/ml) and 27 control subjects (15 females, 12 males; aged 45 +/- 8 years; GH levels 3 +/- 1.5 ng/ml). The patients and controls were matched for age, gender, body mass index, cigarette smoking, blood pressure, lipid levels, diabetes mellitus, hypertension, and its duration. Endothelial function, measured as FMD of the brachial artery using ultrasound, was calculated in the 2 groups. The endothelial function was evaluated by assessing 1-min postischemic FMD of the brachial artery. RESULTS The FMD was lower in patients with acromegaly (9.97 +/- 3.5%) than in controls (16.1 +/- 3.4%), and the difference was statistically significant (p = 0.0001). CONCLUSIONS Endothelial dysfunction may develop in the preclinical phase of atherosclerosis in patients with acromegaly. Endothelium-dependent FMD may be impaired in acromegalic patients, and measurement of endothelial function may identify high-risk individuals earlier.