BACKGROUND: Chronic myocardial ischaemia due to coronary artery stenosis or occlusion has been shown to increase the growth of coronary collateral circulation. Collateralization leads to increased oxygen delivery to the area at risk and hence may reduce ischaemia, prevent infarction and preserve contractile function. However, there is considerable variation among patient subsets in terms of the presence or degree of collateralization. We aimed to evaluate the relationship between obesity and coronary collateral development in patients with ischaemic heart disease. METHODS AND RESULTS:In all, 215 patients (mean age, 57.878.9 y) with body mass index (BMI)Z30 kg/m 2 were enrolled into our study. A total of 90 age-and sex-matched patients (mean age, 58.7710 y) with BMIo25 kg/m 2 and significant coronary artery disease were selected as a control group. The mean age and distribution of risk factors for coronary heart disease were not significantly different between two groups other than poorer HDL cholesterol and triglyceride profile in obese patients. The mean BMI was significantly higher in the patient group (33.372.4 vs 22.871.7, Po0.001). The mean number of diseased vessels and maximum lesion severity were not significantly different between the two groups. The mean Rentrop collateral score of the patient group was significantly worse than the control group (1.0870.68 vs 2.1070.72, Po0.001). CONCLUSIONS: Our findings suggest that collateral vessel development is poorer in obese patients (defined as BMIZ30 kg/m 2 ) with ischemic heart disease compared to normal range BMI, and the risk of having poor collateral vessel development is significantly increased. However, this might be reflecting the cluster of risk factors, associated with metabolic syndrome, in which insulin resistance plays a major role.