OBJECTIVE: To assess the contribution of coronary artery ectasia, either isolated or in association with obstructive coronary artery disease, to morbidity and mortality from ischaemic heart disease. DESIGN: A retrospective study of patients undergoing coronary arteriography at a tertiary cardiac centre. PATIENTS AND METHODS: The epidemiological, clinical, arteriographic, and follow up characteristics of three groups of patients were examined: group A, 172 patients with coronary artery ectasia and coexisting significant coronary artery disease; group B, 31 patients with coronary artery ectasia only; group C, 165 patients with significant coronary artery disease but without ectasia, matched for sex and age with group A. RESULTS: Group A patients had a similar incidence of a previous myocardial infarction to group C patients (61.6% v 64.2%), exercise performance, severity of obstructive lesions (CASS score 2.19 v 2.14), and similar distribution of diseased vessels. At follow up of approximately two years they experienced a similar incidence of unstable angina (7.5% v 4.4%) and myocardial infarction plus cardiac death (4.9% v 6.1%). They underwent bypass surgery with similar frequency (39% v 42%) but there was a lower frequency of percutaneous transluminal coronary angioplasty (5.8% v 17%, P < 0.01). Patients with pure coronary ectasia (group B) had a lower incidence of a previous myocardial infarction (38.7%, 12/31, P < 0.05) than the two other groups. The infarct in all cases was related to an ectatic artery. Their exercise performance and ejection fraction (9 (SD 3) minutes and 56.5(9)%) were higher (P < 0.01) than group A (5 (2) minutes, 48.3(10)%) and group C (5.3 (2) minutes, 49.3(10)%). Group B had no myocardial infarctions, cardiac death, surgery, or intervention at follow up; 4.4% (5/115) developed unstable angina. The incidence of angina at study entry was similar in all three groups (38.7-49.7%). CONCLUSIONS: Coronary artery ectasia does not confer added risk in patients with coexisting obstructive coronary artery disease. Although there is a measurable incidence of previous myocardial infarction, patients with pure ectasia have a good prognosis. The wisdom of giving oral anticoagulants to such patients is questioned.