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To propose a new parameter, "Parallelism," to evaluate retinal layer integrity on spectral-domain optical coherence tomography (SDOCT), and to investigate the association between parallelism and visual function in eyes with idiopathic epiretinal membrane (ERM).Retrospective, observational evaluation of a diagnostic test.We evaluated a consecutive series of 57 eyes of 57 patients with ERM and 30 healthy eyes of 30 volunteers for whom M-CHARTS testing and SDOCT were performed on the same day. OCT images were skeletonized, and the orientation of segmented lines in the image was termed "Parallelism" and was expressed as a value ranging from 0-1 and increasing as the retinal layers ran more parallel with each other. The relationships between parallelism and visual acuity and between parallelism and metamorphopsia score were evaluated.In normal eyes, parallelism was nearly homogeneous and varied slightly with the location. Parallelism in eyes with ERM was significantly lower than that in normal eyes. In the horizontal and vertical scans, parallelism was significantly correlated with visual acuity, horizontal metamorphopsia score, and vertical metamorphopsia score. Parallelism of the center (1 mm) in the horizontal scan was strongly correlated with horizontal metamorphopsia score (R = -0.632; P.0001). Significant negative correlation was found between parallelism and retinal thickness both in horizontal and vertical scans.Parallelism was significantly lower in eyes with ERM than in normal eyes, and correlated strongly with metamorphopsia and visual acuity in eyes with ERM.