Purpose: To compare the efficacy of surgically removing or not removing the internal limiting membrane (ILM) during pars plana vitrectomy on the visual acuity and retinal thickness in eyes with diabetic macular edema. Methods: A prospective, case-control study was carried out on 30 eyes of 29 patients undergoing pars plana vitrectomy for diabetic macular edema. Fifteen eyes underwent pars plana vitrectomy with ILM removal and 15 eyes without ILM removal. Results: In 7 of 15 eyes (47%) in the ILM-removed group, the visual acuity improved by 0.2 or more log of the minimum angle of resolution (log MAR) units and remained unchanged in 8 eyes (53%). In the ILM-preserved group, the final visual acuity improved in 9 of 15 eyes (60%) and remained unchanged in 6 eyes (40%). The difference in visual acuity between the two groups after 11 months the surgery was not significant (Fisher’s exact test, p = 0.4938). In the ILM-removed group, the final retinal thickness decreased by more than 20% of the preoperative retinal thickness in 12 of 15 eyes (80%), remained unchanged in 2 of 15 eyes (13%), and increased in 1 of 15 eyes (7%). In the ILM-preserved group, the final retinal thickness decreased in 13 of 15 eyes (87%) and remained unchanged in 2 of 15 eyes (23%). The differences in the changes in the retinal thickness between the two groups were not statistically significant (Fisher’s exact test, p = 0.5945). Conclusion: Vitrectomy in eyes with diabetic macular edema without ILM removal was as effective in reducing the retinal thickness and improving the visual acuity as eyes with ILM removal. We conclude that ILM need not be removed to treat eyes with diabetic macular edema.