Background Some patients with typical angina and electrocardiographic evidence of ischemia have normal coronary angiograms. These patients have a reduced coronary flow reserve and abnormal endothelium-dependent vasodilator responses; this syndrome is known as microvascular angina. Among endothelium-derived peptides, endothelin-1 (ET-1) is a potent vasoconstrictor and an important modulator of microvascular function. Methods Plasma ET-1 was measured in 13 patients with typical angina, instrumental evidence of ischemia, and normal arteriograms and in 20 normal control subjects. Results Mean concentration of ET-1 was 2.89 ± 1.24 pmol/L in patients with angina and normal angiograms and 1.99 ± 0.81 pmol/L in normal control subjects (p < 0.02). Plasma levels of ET-1 values were significantly higher in patients with angina, positive exercise test results for ischemia, and normal coronary arteriograms compared with the group of patients with no clinical or instrumental evidence of ischemia. Conclusions This is consistent with the hypothesis that in patients with microvascular angina, an endothelial dysfunction in the coronary vascular area caused by impaired endothelium-derived ET-1 could play an active role in the disease process. (Am Heart J 1998;135:684-8.)