Objective Obstructive sleep apnoea (OSA) has been linked with many forms of cardiovascular diseases. Aortic knob width (AKW) has increasingly been presented as a marker of diagnosis and prognosis of atherosclerotic coronary artery disease. We sought to determine whether AKW has an association with sleep apnoea syndrome in patients referred to sleep testing in order to provide baseline information for future studies on general population. Methods A total of 363 patients were included in this retrospective study. The capacity of AKW indexed by patients' height in predicting the presence of severe OSA was analysed using receiver operating characteristics curve analysis. The association of OSA severity with AKW indexed by height was tested with two separate models of ANCOVA (analysis of variance). Results Area under the curve for estimating severe OSA with AKW was 0.67 (95% CI, 0.60 to 0.73, P < 0.001). A cut-off value of 21.42 mm/m provided a sensitivity of 62.6% and a specificity of 70.7% for severe OSA. In both ANCOVA models, the association of OSA severity on AKW indexed by height was significant and the effect size was moderate (P < 0.001, partial eta squared: 0.09 and 0.05). Conclusions Aortic knob width indexed by height has a significant association with the severity of OSA; however, the association is significantly influenced by gender and age.