Ketanserin is a selective 5-hydroxytryptamine (5-HT2) antagonist with vasodilator properties in the systemic and pulmonary circulation. Ketanserin also can inhibit serotonin-induced coronary artery vasoconstriction during percutaneous transluminal coronary angioplasty (PTCA). The in vivo effect of ketanserin on the coronary arteries of patients with stable angina has not previously been reported. The effects of intravenous ketanserin on cardiac haemodynamics and coronary artery diameter were measured in 10 patients with stable angina undergoing diagnostic cardiac catheterisation. Ketanserin (10 mg, i.v.) was associated with significant reductions in systemic and pulmonary arterial pressure (p < 0.05) and total systemic (SVR) and pulmonary (PVR) vascular resistance (p < 0.05). No significant change in mean coronary artery diameter or coronary artery stenotic index was evident after ketanserin. Vasodepressor responses in the systemic and pulmonary arterial circulation were observed after ketanserin injection. We assume these responses to be a direct effect of ketanserin, although non-drug-induced changes over time cannot be excluded. No significant effect on coronary artery diameter was observed, presumably because circulating serotonin levels are low in patients with stable anginal symptoms.