Slow coronary flow is an angiographically diagnosed phenomenon defined as delayed opacification of epicardial arteries in the absence of significant arterial narrowing and blockade. Endothelial dysfunction at the level of microarteries have been proposed as the main pathological mechanism in this regard. Available evidence suggest that standard anti-angina medications (e.g. nitroglycerin) that solely target large coronary trunks might not provide adequate symptomatic relief in patients with slow coronary flow phenomenon. It is hypothesized that anti-angina medications which exert vasodilatory effects in large coronary arteries as well as small dividing branches might be superior to nitroglycerin in amelioration of angina symptoms. The present randomized clinical trial was thus designed and conducted to compare the short-term efficacy of nicorandil (a dual-acting anti-angina medication with effects on both large and small coronary vessels) with nitroglycerin in a group of patients with slow coronary flow presented with frequent angina episodes.