Morbidity of patients with cardiac syndrome X (typical anginal-like chest pain and normal coronary arteriograms) is high with continuing episodes of chest pain and frequent hospital readmissions. Management of this syndrome represents a major challenge to the treating physician. Evidence for the important role of endothelial dysfunction and oxidative stress in the pathogenesis of cardiac syndrome X has been recently strengthened by the finding that basal superoxide production predicts future cardiovascular events in this patient group. The investigators have recently shown that high-dose allopurinol abolishes vascular oxidative stress and improves endothelial function in patients with chronic heart failure, which makes allopurinol a prime candidate to reduce oxidative stress in syndrome X. The hypothesis to be tested in this study is whether allopurinol offers dual benefits of improving vascular function and reducing myocardial ischaemia in patients with cardiac syndrome X. This study may discover a novel way to improve endothelial function and anginal symptoms which are often debilitating in these patients.