OBJECTIVE To test the hypothesis that the active coronary endothelial lesions in unstable angina raise the endothelin concentration in coronary venous blood. METHODS Systemic and coronary venous blood samples were obtained from unselected patients with the clinical syndromes of either stable or unstable angina at the time of cardiac catheterisation and coronary arteriography. Control venous blood samples were obtained from healthy laboratory workers and from patients with chronic renal failure treated by intermittent haemodialysis. METHODS Twelve patients with angina: seven with stable symptoms and five with unstable angina. RESULTS The mean coronary venous endothelin concentration in unstable angina was 2.32 ng/l (range 1.7-3.2 ng/l). In stable angina it was 2.77 ng/l (range 2.1-4.4 ng/l). These values were not significantly different from one another nor from the values obtained in systemic venous blood from either group or from the healthy controls. Circulating endothelin concentrations were much higher in venous blood from the patients treated by haemodialysis. CONCLUSIONS These data do not support the hypothesis that raised endothelin concentrations in coronary blood in patients with unstable angina may modulate variations in coronary arterial tone thereby contributing to the clinical syndrome of chest pain and electrocardiographic changes at rest. The raised endothelin concentrations seen in systemic venous blood after myocardial infarction may be part of the systemic response to myocardial infarction.