BACKGROUND AND OBJECTIVES: Bilirubin has a protective role in suppressing atherosclerosis and coronary artery disease by its potent physiological antioxidant properties. There has been no comparative study on the relation between the bilirubin level and the coronary microvascular function in diabetic patients. This study investigated whether the bilirubin level correlates with the coronary microvascular integrity in diabetes by assessing the coronary flow velocities after successful percutaneous coronary intervention (PCI). SUBJECTS AND METHODS: Fifty patients (31 males and 19 females, mean age 60+/-11) with angina and who received elective PCI were studied. Using an intracoronary Doppler wire, the coronary flow velocity reserve (CFR), the hyperemic microvascular resistance index and the phasic coronary flow velocity patterns were measured after PCI. RESULTS: The mean value of the fasting blood glucose was 211+/-88 mg/dL, the man value of glycated hemoglobin A1c (HbA1c) was 8.1+/-1.6% and the mean serum total bilirubin level was 0.59+/-0.21 mg/dL. CFR was significantly correlated with the serum bilirubin level (r=0.485, p<0.001), HbA1c (r=-0.432, p=0.003) and the fasting blood glucose (r=-0.361, p=0.011). On multivariate analysis, HbA1c, bilirubin and left ventricular hypertrophy showed independent relationships with coronary microvascular dysfunction (p=0.003, p=0.004, p=0.033, respectively). CONCLUSION: These results suggest that glycemic control and elevated serum bilirubin may protect diabetic patients from coronary microvascular dysfunction.