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Background The optimal selection of stent size remains uncertain. This study evaluated the impact of stent diameter on in-stent stenosis (ISS) after interventional therapy through a subgroup analysis of the Intracranial Aneurysms Managed by Parent Artery Reconstruction Using Tubridge Flow Diverter (IMPACT) trial.Methods This prospective, multicenter study included patients treated with the Tubridge Flow Diverter (TFD) at 14 centers between December 16, 2019 and October 26, 2022. The primary outcome was the incidence of ISS at the 12-month follow-up. We analyzed the effect of different stent sizes on ISS in all patients from IMPACT who had follow-up imaging. The parent artery was divided into proximal and distal vessels, and we introduced the ratio of diameter (RD) as a parameter to assess the association between stent diameter and vessel diameter.Results A total of 167 patients with 195 intracranial aneurysms were included. ISS was classified as mild (<25%) in 9 cases (5.4%), moderate (25–50%) in 4 cases (2.4%), severe (51–75%) in 1 case (0.6%), and extremely severe (>75%) in 5 cases (3%). A total of 390 proximal and distal vessels were evaluated, with a mean RD of 1.14±0.26. Smaller parent artery diameter and higher RD were significantly associated with ISS (p=0.002 and p=0.021, respectively). Additionally, the risk of ISS increased when RD>1.1 (p=0.006).Conclusions In TFD treatment for intracranial aneurysms, stent size and the diameters of proximal and distal parent arteries should be carefully considered, especially in cases involving small parent arteries. Maintaining an RD of less than 1.1 may reduce the risk of ISS.