Objective: Collateral channel opening is one of the components of the ischemic tolerance developing during subsequent coronary balloon occlusions. The effect of diabetes mellitus (DM) on coronary collateral recruitment (CR) is still not known. We therefore sought the effect of DM on CR in patients with stable angina pectoris (SAP) by using intracoronary pressure measurement technique. Methods: Study material consisted of 44 patients (21 diabetic) with SAP. All of the patients had single vessel disease with more than 70% vessel narrowing and all of them underwent stent implantation to this vessel. After angiography, fiber-optic pressure monitoring guide-wire was advanced distal to the stenosis to be dilated. Myocardial fractional flow reserve (FFRmyo) was determined under adenosine hyperemia by the ratio of simultaneously measured mean distal pressure to mean aortic pressure. During subsequent two 1 minute balloon occlusions, distal pressures were recorded as coronary wedge pressure (CWP). Collateral flow index was determined by the ratio of simultaneously measured CWP to mean aortic pressure. Percentage of the improvement in the coronary flow index (CFI) (first to second occlusion) between two occlusions was determined for each patient. Results: There was no difference between two groups in terms of pre-intervention FFRmyo (0.54 ± 0.12 in DM group and 0.50 ± 0.11 in non-DM group). The baseline CFI was significantly higher in non-DM group (0.26 ± 0.09 versus 0.17 ± 0.08, p<0.03). Beyond this finding, mean CFI increased by 17% (from 0.17 ± 0.08 to 0.20 ± 0.09) in DM group and by 30 % (from 0.26 ± 0.09 to 0.34 ± 0.10) in non- DM group. There was statistically significant difference between these two groups in terms of improvement in CFI during subsequent balloon occlusions (p<0.01). Conclusion: In addition to poor collateral vessels seen in patients with DM, CR is also impaired. This finding suggests that DM abolishes ischemic tolerance in terms of CR as well.