OBJECTIVE Undiagnosed diabetes mellitus (DM) or impaired glucose tolerance (IGT) is not uncommon in patients with coronary atherosclerosis and is known to be associated with abnormal scores for atherosclerosis surrogates. We sought to determine the prevalence of undiagnosed DM or IGT, and the association between insulin resistance (IR) and atherosclerosis surrogates in patients with coronary atherosclerosis. METHODS The study population consisted of 187 consecutive patients with angiographically proven coronary atherosclerosis (mean: 61 years old, 94 males). We measured carotid intima-media thickness (IMT) and flow mediated brachial artery dilatation (FMD). We also did oral glucose tolerance tests (OGTT), quantitative insulin-sensitivity check indexes (QUICKI) and homeostasis model assessment-IR (HOMA-IR). RESULTS Abnormal OGTT was found in 164 patients (87.7%), even though there were only 63 known cases of DM (33.7%). There were 58 patients (31%) with newly diagnosed IGT and 43 patients (23%) with newly diagnosed DM. There were 71 patients (38%) who had IR (defined as measured HOMA-R >/=3.0). HOMA-IR showed a positive correlation with body mass index (BMI) (r=0.275, p<0.001) and triglycerides (r=0.2, p=0.01), whereas QUICKI had a negative correlation with BMI (r=-0.26, p<0.001), total cholesterol (r=-0.15, p=0.04), triglycerides (r=-0.21, p=0.004) and low-density lipoprotein-cholesterol (LDL-C) (r=-0.17, p=0.02). HOMA-IR and QUICKI were not significantly correlated with IMT or FMD. CONCLUSIONS This study suggests that there is a high incidence of undiagnosed DM and IGT, but atherosclerosis surrogates are not associated with IR in patients with coronary atherosclerosis.