Syndrome X is the term applied to patients with anginal-type chest pain who, despite a positive exercise stress test, have angiographically normal coronary arteries. Such patients probably belong to a heterogeneous group, and in a proportion the exercise test is falsely positive. With the availability of more accurate techniques for the detection of myocardial ischaemia, it is apparent that some patients can be shown to develop transient myocardial ischaemia with stress. The paradox of normal coronary arteries and a positive exercise test may be resolved by improved understanding of the regulatory control of regional myocardial blood flow, particularly at the level of the microvasculature, and of the metabolic expression of myocardial ischaemia.