Invasive investigation of coronary artery disease is relatively expensive, and carries risks including a mortality of approximately 1 in 2000. It would not be practical or appropriate to perform invasive investigation in all patients with a clinical diagnosis of coronary artery disease, still less in the large numbers with chest pain and possible angina. Clinicians will refer for invasive investigation those: (i) with a high level of angina, needing revascularisation on symptomatic grounds; and (ii) who are likely to have a poor prognosis with medical treatment, and thus likely to benefit from revascularisation. Not all of these patients will have a high level of symptoms.