Standard treatment for severe coronary artery disease (CAD) is coronary artery bypass grafting (CABG). An underreported branch of the internal mammary artery, the lateral costal artery (LCA), can cause a steal phenomenon after CABG, resulting in angina. The aim of this study was to determine the prevalence and length of LCA based on CT angiography (CTA). This retrospective study included adult patients undergoing a thoracic CTA between January 2016 and August 2018. Exclusion criteria were prior CABG, insufficient clinical information, or inadequate image quality. Two blinded, independent readers reviewed all studies for the prevalence of the LCA. Positive cases were reviewed by two readers (R1/R2) for side distribution and vessel length, measured in intercostal spaces (ICS). Study indication, aortic size, and coronary calcification were noted. LCA was present in up to 42/389 (11%) of studies (60.3 ± 16.7 years, 30 males). The LCA was most commonly unilateral (n = 23, 55%). Median vessel length was 2 ICS (IQR 0; 3). Logistic regression was not significant in vessel distribution for sex (OR 0.6, 95% CI 0.28-1.15; p = 0.11). Inter-observer agreement in detecting LCA was substantial (kappa 0.71, 95% CI 0.59-0.83) and excellent for side/length distribution (kappa 0.94, 95% CI 0.82-1.0; ICC 0.96, 95% CI 0.93-0.98). The LCA is uncommon and most often unilateral and extends the third rib. Radiologists should be aware of this vessel and its potential role in angina after CABG, particularly when large. • LCA is an uncommon normal variant that is reported to cause angina pectoris after CABG. • CT angiography can reliably detect the LCA. It is most often unilateral and spans two intercostal spaces.