Objective: To evaluate retinal nerve fiber layer (RNFL) thickness changes of type 1 diabetes mellitus (DM) patients with and without diabetic retinopathy (DR). Material and Methods: One hundred-eighteen patients with type 1 DM and 49 age-matched control subjects were enrolled in the study. Ninetyeight of 118 diabetic patients with DR were as signed as group 1 and there maining 20 patients without DR were labeled as group 20 (n= 98). The RNFL thickness of all subjects were measured using optical coherence to mography (OCT). Results: The study included 118 type 1 DM patients with a mean age of 18.77±8.80 ye ars. The mean age of 49 subjects in the control group was 18.71 ± 5.72 (range 7-40) years.The mean RNFL thickness was 103.79 ± 6.45 μm in the control group, 100.00 ± 11.93 μm in group 1 and 85.59 ± 19.81 μm in group 2. The decrease in group 2 was found statistically significant (p< 0.001). The mean RNFL thickness in the superior, nasal, inferior and tem poral quadrants were less in group 1 compared to controls, however this data was not statistically significant (p= 0.274, p= 0.149, p= 0326, p= 0783, respectively). In group 2, the RNFL thickness revealed a statistically significant decrease in all quadrants (p< 0.001) except tem poral quadrant (p= 0.396). The mean duration of DM was significantly longer in group 2 (153.80 ± 70.35) compared to group 1 (60.76 ± 50.41 months) (p< 0.001). However, there was no correlation between the RNFL thickness and the durati on of DM. Conclusion: The RNFL thickness of patients with type 1 DM was found less compared to control subjects. This was more prominent in patients with established retinopathy (gro up 2). The sefin dingssuggested that the RNFL thickness measurement with the aid of OCT may be used as an adjunctive diagnostic tool for early diagnosis of DR. © 2011 by Türkiye Klinikleri.