['Thymosin beta 4 (Tβ4) plays an essential role in cardiac vessel development and is currently being developed as a therapeutic agent for the treatment of coronary artery disease (CAD) in some experimental studies. Thus, we aimed to investigate the association of serum Tβ4 levels and collateral formation in patients presenting with severely stenotic CAD.', 'Thirteen patients with poor collateral development and 16 age- and sex-matched patients with good collateral development who had ≥ 95% stenosis in at least one major coronary artery on coronary angiogram (CAG) were enrolled in the study. The Gensini score was calculated for each patient by using CAG results. Collateral development was classified according to the Cohen-Rentrop method. Serum Tβ4 levels were measured with enzyme-linked immune sorbent assay.', 'There were no statistically significant differences between the two groups in regard to clinical and laboratory characteristics of the patients except for Tβ4 levels. The Tβ4 levels in the well-collateralized study group were found to be significantly higher than those of the poorly collateralized study group and serum Tβ4 levels were positively correlated with the collateral development.'] Is thymosin beta 4 associated with collateral development in coronary artery disease?