Serotonin is released by activated platelets and may act as a mediator to initiate or sustain certain unstable syndromes of ischemic heart disease in humans. To determine whether or not serotonin concentration increases across the coronary bed in patients with severe, limiting angina, we measured central aortic and coronary sinus serotonin concentrations by a sensitive radioenzymatic assay in 39 patients with coronary artery disease and 13 patients with minimal or no coronary artery lesions as detected by arteriography. Although no difference existed in the mean aortic or coronary sinus serotonin concentrations between these two groups, elevated coronary sinus serotonin concentrations were detected in 23% of those with coronary artery disease. The coronary sinus and aortic serotonin concentration difference was greater in patients with significant coronary artery disease (0.6 +/- 6.62 ng/ml) compared with patients without significant coronary artery disease (-5.6 +/- 10.32 ng/ml) (mean +/- SD) (p less than 0.05). Further analysis revealed that patients with eccentric, irregular coronary artery lesions or intraluminal filling defects had a significantly elevated coronary sinus and aortic serotonin difference (3.1 +/- 5.54 ng/ml) compared with those with smooth concentric lesions (-1.9 +/- 6.61 ng/ml) (p less than 0.02). These data suggest that serotonin is released into the coronary circulation of some patients with coronary artery disease, especially those with frequent angina and complex coronary lesions. Although serotonin may be released in some patients with coronary artery disease, the specific pathophysiologic role of serotonin in the development or perpetuation of certain coronary syndromes in humans remains to be determined.