OBJECTIVE : To investigate the real incidence of type IIIb endoleaks associated with the Zenith stent graft, currently the most widely used third-generation polyester stent graft, in a Japanese multicenter database. METHODS : Retrospective analysis was conducted of 433 patients who underwent endovascular aneurysm repair (EVAR) of aortoiliac aneurysms with the use of Zenith stent grafts from 2007 to 2016. The mean age of patients was 75.6 y ± 7.8, and 85.2% of patients were men. Mean transverse diameters of abdominal aortic aneurysms and common iliac artery aneurysms were 50.8 mm ± 9.9 and 42.2 mm ± 10.3, respectively. RESULTS : During a mean follow-up period of 41.9 mo, 7 type IIIb endoleaks (1.6%) were identified. Four patients were definitively diagnosed during repeat intervention, and 3 were diagnosed based on postoperative CT images. Three patients were treated surgically, 2 were treated by relining with an additional stent graft, and 2 were conservatively followed with CT imaging. Only 1 of 5 patients who underwent repeat intervention had a preoperative diagnosis of type IIIb endoleak, reflecting the difficulty in its diagnosis. CONCLUSIONS : The incidence of type IIIb endoleak in Zenith stent grafts was 1.6% in this study, suggesting that type IIIb endoleaks are not extremely rare. In addition, type IIIb endoleak may be underdiagnosed or misdiagnosed as a different type of endoleak. Type IIIb endoleaks should always be considered in patients with sac expansion after EVAR.