BACKGROUND Patients with acute myocardial infarction and single-vessel disease may have early atherosclerosis in other angiographically normal coronary arteries. METHODS Coronary endothelial responses were analysed in 20 non-diabetic patients with an acute myocardial infarction and one-vessel disease. In an angiographically normal, non-infarct related, coronary artery serial acetylcholine doses of 10(-7) M, 10(-6) M, 10(-5) M and 10(-4) M and nitroglycerin 40 micrograms were infused over 3 min. The responses of the coronary vessel were measured with quantitative angiography. Coronary blood flow was also measured with a Doppler catheter in 11 of the 20 patients. RESULTS Four patients showed a trend towards vasodilation during acetylcholine infusion in the proximal and distal segments: from 2.49 +/- 0.23 mm to 2.95 +/- 0.42 mm and 2.43 +/- 0.56 mm to 2.81 +/- 0.66 mm, respectively. Coronary vascular resistance decreased to 57 +/- 4% (P = 0.03). The other 16 patients presented vasoconstriction in the proximal and distal segments: 2.61 +/- 0.75 mm to 2.03 +/- 0.65 mm (P = 0.0001), and 2.40 +/- 0.58 to 1.81 +/- 0.56 mm (P = 0.0036), respectively. Nitroglycerin caused vasodilation in the proximal (2.69 +/- 0.61 mm, P = 0.017, ANOVA) and distal segments (2.48 +/- 0.45 mm, P = 0.009, ANOVA). Coronary vascular resistance increased to 141 +/- 43% (P = 0.03) over the basal value in this group of patients. CONCLUSIONS Endothelial dysfunction of the epicardial and resistance vessels was found in angiographically normal coronary arteries of patients with one-vessel disease in 75% of this population.