Background: Eosinophils (EOS) have been associated with prognosis of patients with coronary artery disease, and those who showed plenitudinous coronary collateral circulation (CCC) often have good clinical consequences. However, the relationship between EOS and CCC was seldom reported. Objective: To investigate the relationship between EOS and CCC development in patients with unstable angina pectoris (UAP). Methods: The study population consisted of 502 consecutive patients with UAP who underwent coronary angiography and coronary stenosis >= 80%. CCC was graded according to the Rentrop grading system of 0-3. Rentrop grades of 0 and 1 indicated low-grade CCC group, whereas grades 2 and 3 indicated high-grade CCC group. Results: The EOS was significantly higher in the high-grade CCC group compared with the low-grade CCC group. In multiple logistic regression analysis, EOS (odds ratio: 1.969; 95% confidence interval [CI]: 1.210-3.3205; P = 0.006) and neutrophil count (odds ratio: 0.757; 95% CI: 0.584-0.981; P = 0.035) were predictors of high-grade CCC development. EOS of >0.12x10(9)/L could independently predict high-grade CCC with 72.5% sensitivity and 58.4% specificity (area under the curve: 0.681; 95% CI: 0.632-0.729). Conclusion: EOS were associated with high-grade CCC in patients with UAP with coronary stenosis >= 80%. Increased EOS count may play an important role in the development of CCC in patients with UAP.