So far, there has been no closure grade system synthesizing morphological and microstructural features for large idiopathic macular holes (IMHs) treated by vitrectomy and internal limiting membrane (ILM) peeling. This study aimed to propose a concise one and explore its relevance with visual acuity and the related preoperative factors.Consecutive patients with large IMHs (minimum diameter >400 μm), undergoing vitrectomy and ILM peeling, obtaining primary closure and regularly followed-up were enrolled. Preoperative clinical charts and spectral-domain optical coherence tomography (SD-OCT) parameters were reviewed. SD-OCT images and best corrected visual acuity (BCVA) were assessed at 1, 4, and 10 months postoperatively. SD-OCT features at last visit were categorized by BCVA significance, and preoperative risk factors were analyzed.Sixty-eight eyes from 64 patients were enrolled. The 10-month postoperative SD-OCT images were categorized into closure grade 1, 2, and 3 with successively decreased BCVA (p < 0.001). During early follow-up, part of grades 2 and 3 could evolve into the upper grade, respectively, but grade 3 could never evolve into grade 1 and exhibited the least satisfactory long-term BCVA. Binary logistic regression showed that large minimum linear diameter (MLD) was a risk factor for grade 3 occurrence (p < 0.001), with a cutoff value of 625.5 μm from the receiver operating characteristic curve for MLD predicting grade 3 occurrence (p = 0.001).Long-term closure status of large IMHs could be categorized into three grades with BCVA significance. Large horizontal MLD is a risk factor for occurrence of grade 3 closure with unsatisfactory visual recovery.© 2024 The Author(s). Published by S. Karger AG, Basel.