Objective: Impaired cholinergic vasodilatation in the forearm in hypertension and hypercholesterolemia has been attributed to impaired nitric oxide bioavailability. However, inhibition of cyclooxygenase reverses the impaired cholinergic dilatation in hypertensive animals and patients. The aim of this study was to examine the effect of aspirin on cholinergic vasodilatation in hypercholesterolemic patients. Methods: We examined responses to brachial artery infusion of acetylcholine and the endothelium-independent vasodilator sodium nitroprusside in the presence or absence of aspirin in 10 hypercholesterolemic patients (7 men/3 women; aged 38–63 yr; systolic blood pressure 133±5 mmHg; diastolic blood pressure 80±3) compared with 10 matched controls (7 men/3 women; aged 38–63 yr; systolic blood pressure 126±2; diastolic blood pressure 77±2). Results: In hypercholesterolemic patients, forearm vasodilatation was impaired in response to acetylcholine (112±20 vs. 346±30% increase in blood flow in controls, at the highest dose [15 μg min−1]; P<0.0001) but not in response to sodium nitroprusside. With the addition of aspirin, baseline forearm blood flow was unaltered. However, forearm vasodilatation to acetylcholine was partially restored in hypercholesterolemics (from 112±20 to 193±30%; P<0.001) though not affected in controls. Vasodilator responses to sodium nitroprusside were unaffected by aspirin in either group. Conclusions: In hypercholesterolemia, an altered balance between vasoconstrictor and dilator prostanoids, favouring constrictors, may contribute to endothelial dysfunction either directly or through an effect on nitric oxide synthesis. Restoration of this imbalance may be a component of the therapeutic benefit of aspirin in cardiovascular disease.