Prostacyclin and its stable analog iloprost (ILO) are able to induce vasomotor effects in smooth muscle cells. These vasodilator effects have been demonstrated in several types of vessels, even in coronary arteries. Bugiardini et al. have previously claimed to have evidence for a coronary steal phenomenon in patients with coronary artery disease receiving ILO. They conclude the steal phenomenon from the appearance of angina and ST-segment changes. The aim of our trial was to study the coronary circulation by measuring coronary vessel diameter, coronary blood flow velocity (CBFV), heart rate and blood pressure in patients with high degree LAD stenosis following i.v. and i.c. infusion of ILO and papaverine (PAPA i.c.). Infusion of ILO i.v. (2 ng kg-1 min-1) induced no significant change in vessel diameter measured in the stenotic and in normal vessel segments. Following ILO i.c. (0.5 ng kg-1 min-1) and PAPA coronary vessel diameter increased proximal to the stenosis by 25.8 +/- 5.6% and 24.8 +/- 4.6% and within the stenosis by 50.1 +/- 10% and 56.2 +/- 11%, respectively (p < 0.01). At the end of the i.c. ILO infusion CBFV increased by 103.7 +/- 24.9% and was further elevated by PAPA (217 +/- 57.2%, p < 0.01). The elevated flow velocity with increased vessel diameter following ILO i.c. (0.5 ng kg-1 min-1) implicated an increased flow in the stenotic vessel and thus there was no evidence for a decreased perfusion or coronary steal phenomenon.