Background: Atherosclerosis of the aorta has been associated with coronary artery disease. There are few data about the influence of aortic knob calcification on the severity of coronary artery disease and its relationship with coronary plaque composition. We evaluated differences in plaque composition and clinical significance in coronary artery disease according to the presence or absence of aortic knob calcification using coronary CT angiography (CTA). Methods: A total of 452 consecutive coronary CTA examinations were performed between July 2007 and June 2008. Of these, 55 patients underwent invasive coronary angiography because they have significant coronary artery disease. The aortic knob calcification was assessed via chest posteroanterior view. On coronary CTA, plaques were classified into soft plaque, calcified plaque, and mixed plaque. Significant coronary narrowing was defined as > 60% luminal narrowing. Results: Aortic knob calcification was present in 20 patients (36.4%). Presence of coronary plaque (100% vs. 80%, p=0.040), multi-vessel disease (65% vs. 28.6%, p=0.012), critical coronary stenosis (40% vs. 11.8%, p=0.013), and AHA/ACC B2/C lesions (80% vs. 37.1%, p=0.004) were higher in patients with aortic knob calcification than in patients without calcification. The plaque distribution was as follows: 5.0% soft plaques, 40.0% calcified plaques, and 55.0% mixed plaques in patients with aortic knob calcification vs. 39.9% soft plaques, 25.0% calcified plaques, and 35.7% mixed plaques in patients without calcification (p=0.025). Conclusions: Aortic knob calcification in patients with coronary artery disease can be a marker of more severe coronary artery disease and it may also provide information about coronary atherosclerotic plaque composition.