In this study we tested whether Ca++ has a similar part in the coronary vasomotion of patients with syndrome X (n = 29) and patients with Prinzmetal angina pectoris (n = 12). Through quantitative angiography we evaluated the influence of nifedipine (10 mg sl) on the lumen diameter of proximal, mid and distal thirds of normal epicardial branches. The main differences in the coronary vasomotor reaction were: uniform vasodilatation in Prinzmetal angina and a variable response in syndrome X; greater increase in the coronary lumen in patients in the former as compared to those in the latter group who showed vasodilatation. The variable response in syndrome X was not related to changes in diastolic pressure and cardiac output. Patients showing coronary constriction were those who responded to nifedipine with a higher degree of tachycardia; this might suggest a neural participation in the paradoxical reaction to this drug. In the Prinzmetal group, on the contrary, at similar heart rate increase the pattern was invariably vasodilatation. Thus, calcium ions appear to have a different role in the coronary smooth muscle contractility of the 2 series of patients; in fact, in Prinzmetal angina nifedipine relaxed the coronary arteries more potently and made them unresponsive to stimuli which were still able to cause vasoconstriction in syndrome X.