The left internal mammary (LIMA) is an ideal conduit for bypassing the left anterior descending artery (LAD) during coronary artery bypass surgery (CABG). In this study, we evaluate the frequency of significant disease in the LIMA prior to CABG and describe associated significant lateral costal branches (> 1.5 mm), subclavian disease and vertebral disease. In 115 consecutive patients referred to CABG, 101 patients met the inclusion criteria. All patients who underwent routine visualization of the LIMA vessels were reviewed and quantitatively analyzed by an independent reader. Disease in the LIMA, left subclavian and vertebral arteries were graded and considered significant if lesions were > 50%. Lateral costal branches of the LIMA were also noted for their presence and size. Of the 101 patients, 87.1% of the LIMAs prior to surgery were without any disease. One LIMA had a 25-50% narrowing and the remaining LIMAs showed 0-25% disease. Of the LIMAs, 48.5% had lateral costal branches with diameters > 1.5 mm. The incidence of significant left vertebral disease and proximal subclavian disease was 37.6% and 5%, respectively. Cardiolite stress imaging post-CABG (n = 75) showed that when ischemia persisted in the anterior wall in patients with a LIMA to the LAD, this was not associated with the presence of significant LIMA disease, lateral costal branches, or subclavian disease identified pre-operatively. We conclude that the LIMA rarely shows significant disease on routine visualization prior to CABG. Asymptomatic subclavian artery disease and the presence of lateral costal branches pre-operatively did not correlate with post-CABG ischemia in the LAD territory.