The purpose of this study was to define the morphologic differences in idiopathic and secondary epiretinal membranes (ERMs) using a time domain optical coherence tomography. The medical records and optical coherence tomography images of 293 eyes of 236 consecutive patients diagnosed to have ERM were evaluated retrospectively. Demographic features, best-corrected visual acuities, central macular thickness, membrane attachment patterns, macular changes, and the presence of posterior retinal detachment at the time of diagnosis were compared between the groups (idiopathic group, n = 125 eyes; secondary group, n = 168 eyes). The mean age was higher among idiopathic ERMs (67 ± 9 vs. 62 ± 10 years, P = 0.001). In the secondary group, best-corrected visual acuity was estimated to be worse (P < 0.001) and central macular thickness higher (P = 0.02) than the idiopathic group. In both groups, ERMs were mostly diagnosed at Grade 1 level. Diffuse attachment was more common in both of the groups. Among macular changes, only cystoid macular degeneration differed significantly between the groups (P = 0.0001). Idiopathic and secondary ERMs were estimated to differ significantly at the time of diagnosis in terms of age, visual acuities, macular thickness, presence of cystoid macular degeneration, and posterior vitreous detachment. These parameters may assist the retinal surgeon in the treatment process of ERM.