PURPOSE: To investigate the relationship between the severity of metamorphopsia and the foveal microstructure measured with spectral-domain optical coherence tomography (SD-OCT) in patients with epiretinal membrane (ERM). METHODS: This study included 48 eyes of 48 patients with idiopathic ERM and 18 age-matched normal controls. We examined the logarithm of the minimum angle of resolution best-corrected visual acuity (logMAR BCVA) and the severity of metamorphopsia using M-CHARTS. Central foveal thickness (CFT), central retinal thickness at the fovea (CRT-1mm) and parafovea (CRT-3mm), and macular volume (MV) were measured with SD-OCT software. Based on the obtained OCT image, we divided the 1.0 × 1.0-mm area centered on the fovea into nine sections and quantified the following parameters using an image-processing program: mean thickness of the ganglion cell layer (GCL), inner nuclear layer (INL), and outer retinal layer (ONL+OPL: outer nuclear layer and outer plexiform layer), the degree of the photoreceptor inner and outer segment junction (IS/OS), and external limiting membrane (ELM) disruption. RESULTS: CFT, CRT-1mm, CRT-3mm, MV, mean GCL, INL, and ONL+OPL thickness were significantly larger in patients with ERM than in normal controls. Multiple regression analysis revealed that the severity of metamorphopsia was significantly related to the mean INL thickness (P < 0.0001). LogMAR BCVA had a significant correlation with the degree of IS/OS disruption (P < 0.05), whereas other parameters were not relevant. CONCLUSIONS: In patients with idiopathic ERM, the degree of metamorphopsia is associated with INL thickness, and IS/OS status influences visual acuity.