OBJECTIVE To explore the relationship between plasma microRNA126 (miR-126) level and coronary collateral circulation (CCC) formation and to determine whether the miR-126 in plasma could serve as a blood-based biomarker for CCC in patients with severely narrowed coronary arteries (CAD). METHODS In this prospective study, a total of 120 consecutive CAD patients with ≥ 95% stenosis in one epicardial coronary artery were enrolled. Thirty healthy people served as normal control. They were divided into two groups according to Rentrop grades: patients with grade 2 and 3 collateral development (good CCC group, n = 64) and patients with grade 0 and 1 collateral development (poor CCC group, n = 56). Plasma miR-126 was measured by RT-PCR and serum VEGF was evaluated by ELISA method. RESULTS Fasting plasma glucose (FPG) was significantly lower in patients with good CCC than in patients with poor CCC ((5.99 ± 1.48) mmol/L vs. (6.40 ± 2.50) mmol/L). Plasma miR-126 levels and VEGF levels were significantly lower in CAD patients than in healthy people (0.04 ± 0.01 vs. 0.07 ± 0.02, P = 0.023 and (2 110 ± 455) ng/L vs. (2 574 ± 450) ng/L, P = 0.011, respectively). miR-126 and VEGF levels were significantly higher in good CCC group than in poor CCC group (miR-126: 0.06 ± 0.02 vs. 0.03 ± 0.01, P = 0.021;VEGF:(2 549 ± 614) ng/L vs. (1 759 ± 452) ng/L, P = 0.008) . In CAD patients with good CCC, the miR-126 level was positively correlated to the VEGF expression (r = 0.712, P = 0.005) while there was no correlation between miR-126 level VEGF in CAD patients with poor CCC (r = 0.342, P = 0.483) . Multivariate analysis revealed that plasma miR-126 (OR = 2.145, 95% CI 1.691-2.988, P = 0.001) and VEGF (OR = 1.279, 95% CI 1.068-2.295, P = 0.013) were independent predictors of collateral formation in patients with severely narrowed coronary arteries. In CAD patients, the area under the miR-126 ROC curve is 0.951 (P = 0.002). CONCLUSIONS Plasma miR-126 level is positively correlated to the CCC formation and is an independent predictor of CCC development in patients with severely narrowed coronary arteries, suggesting that plasma miR-126 might be a useful new, stable blood biomarker for predicting CCC formation in patients with severely narrowed coronary arteries.