The status of inflammation may affect the collateral development in patients with diabetes mellitus (DM). Monocytes were found to have an important role in collateral growth in animal studies. We aimed to investigate the possible association of circulating monocyte count with collateral development in patients with DM and severe coronary artery disease (CAD). We enrolled 134 consecutive patients with DM who had a parts per thousand yen95 stenosis in at least one major coronary artery and investigated the relationship between circulating monocyte count and collateral growth. When we analyzed the coronary angiograms of eligible patients, we found that 64 of them had good collateral growth and 70 had poor collateral growth according to the Cohen-Rentrop method. The monocyte count was significantly different between good and poor collateral growth groups (643 +/- A 184 vs. 479 +/- A 143 per mm(3), P < 0.001). In the analysis comparing the Rentrop score with the Gensini score and circulating monocyte count, we found significant correlations (r = 0.293, P = 0.001 and r = 0.455, P < 0.001, respectively). The duration of ischemic symptoms tended to be longer in the good collateral group (1.9 +/- A 4.1 vs. 0.8 +/- A 1.3 years, P = 0.079). The Gensini score was also correlated with the duration of myocardial ischemic symptoms (r = 0.299, P = 0.004). Multivariate analysis revealed an increased monocyte count in the good collateral group [odds ratio (OR), 5.726; 95% confidence interval (CI), 1.817-18.040, P = 0.003, the cut-off value for monocyte was defined as 550 cell/mm(3)]. The increased circulating monocyte count in diabetic patients was evidently related to good coronary collateral growth. This finding may be potentially important in clinical and basic cardiovascular medicine.