Background:Retinal ganglion cell layer (RGC) layer has a very significant role in collecting the visual input from the retina and transmitting the electrical signals to the brain for the creation of a visual image. Unfortunately, that image is affected by many illnesses and type 1 diabetes mellitus (T1DM) is considered one of the commonest chronic illnesses in the world and in Egypt specifically. Aim and objectives: To assess the impact of diabetes mellitus type 1 on the ganglion cell layer, which may help in early detection and prevention of diabetic retinal complications and hence improve the quality of vision among diabetic patients. Subjects and methods:This case-control trial included 100 eyes of 50 cases separated to fifty eyes of diabetic persons and the same eyes of healthy control. The study was conducted at Department of Ophthalmology, Faculty of Medicine, Suez Canal University. Results: There was a statistically significant alteration amongst the studied groups as regard optical coherence tomography (OCT) of eyes, correlation between duration of DM, (Comparison between ganglion cell layer (GCL) thickness in OCT and level of DM control) and correlation between severity and duration of diabetes and OCT measurements. Conclusion: Diabetes has an early impact on the organisation of the neuronal retinal layers macular ganglion cell complex (GCC), which in most of the patients under study even occurred before retinal vascular alterations in addition to vision impairment. Most of the GCC quadrants were statistically significantly thinner in diabetic type I patients compared to healthy controls.