OBJECTIVES: Aortic valve neocuspidization, or the Ozaki procedure, is an alternative to prosthetic valve replacement. However, the association between native aortic root geometry, particularly the sinotubular junction-to-annulus ratio, and postoperative valve competence remains unclear. This study was designed to investigate the association between the sinotubular junction-to-annulus ratio and postoperative aortic regurgitation severity. METHODS: We retrospectively analysed 139 patients who underwent aortic valve neocuspidization between 2012 and 2023. At 1 year, the patients were stratified into either moderate or less-than-moderate aortic regurgitation groups. Intraoperative sinotubular junction and annulus diameters were measured using transoesophageal echocardiography. Logistic regression and receiver operating characteristic analyses were performed. RESULTS: At 1 year, 11 (7.9%) patients developed moderate aortic regurgitation, while 128 (92.1%) had less-than-moderate aortic regurgitation. Patients with moderate aortic regurgitation had higher sinotubular junction-to-annulus ratios (moderate group: median, 1.40; less-than-moderate group: median, 1.19). Receiver operating characteristic analysis showed excellent discrimination, with an optimal cut-off of 1.30. Each 0.1-unit increase in the ratio was associated with a 3.65-fold increase in the risk of developing moderate aortic regurgitation. Preoperative aortic regurgitation severity was associated with univariable analysis outcomes but lost significance after adjustment for the sinotubular junction ratio. CONCLUSIONS: An increased sinotubular junction-to-annulus ratio may be associated with the development of postoperative aortic regurgitation after aortic valve neocuspidization. These findings suggest that the geometric association between the sinotubular junction and the annulus may play a role in valve competence after aortic valve neocuspidization.