To quantify aniseikonia in patients undergoing vitrectomy for epiretinal membrane (ERM) and to investigate the relationship between the aniseikonia and the foveal microstructure by spectral-domain (SD) optical coherence tomography (OCT). Prospective, consecutive, interventional case series. This study included 44 eyes of 44 patients undergoing vitrectomy for idiopathic ERM. We examined visual acuity and aniseikonia using the New Aniseikonia Test and SD-OCT before and 3 and 6 months after surgery. On the basis of the obtained OCT image, we divided the 1.0 × 1.0-mm area centered on the fovea into 9 sections at 0.25-mm intervals and quantified the following parameters using an image-processing program: central foveal thickness and mean thickness of the ganglion cell layer, inner nuclear layer (INL), and outer retinal layer (outer nuclear layer + outer plexiform layer). The status of the photoreceptor inner segment/outer segment junction, external limiting membrane, and cone outer segment tips also was evaluated. Amount of aniseikonia 6 months after surgery. Of 44 patients, 39 (89%) had macropsia, 1 (2%) had micropsia, and 4 (9%) had no aniseikonia preoperatively. Mean preoperative aniseikonia was 6.2% ± 4.5%. Vitrectomy significantly improved visual acuity in patients with ERM but did not change the amount of aniseikonia. Multiple regression analysis revealed that preoperative aniseikonia at 6 months was significantly related to preoperative INL thickness, whereas postoperative aniseikonia at 6 months was associated with postoperative INL thickness at 6 months. Preoperative INL thickness was found to be of significant prognostic value for postoperative aniseikonia at 6 months. Most of the patients with ERM had macropsia. Aniseikonia was not reduced after surgery. The amount of aniseikonia was associated with INL thickness.