Objective: To compare retinal nerve fibre layer thickness (RNFLT) obtained by optical coherence tomography (OCT) between patients with preclinical diabetic retinopathy (DR) and healthy subjects.Study Design: Cross-sectional study.Participants: Ninety-nine type I or II diabetic patients with no visible funduscopic retinal alteration and 77 healthy subjects were included in this study.Methods: All participants were evaluated for peripapillary RNFLT by Stratus OCT. In diabetic patients, levels of blood sugar and glycosylated hemoglobin were examined.Results: The mean RNFLT in diabetic patients and healthy subjects was significantly different (104.2 [SD 10.4] and 108.6 [SD 9.2] µm, respectively; p = 0.004). Compared with the healthy group, the RNFLT in the diabetic group was also significantly less in the superior quadrant and at the 5, 11, and 12 o’clock sectors (p = 0.04, 0.002, and 0.001, respectively). The average RNFLT in diabetic patients with preclinical DR showed a low correlation with fasting blood sugar level (p = 0.03).Conclusions: The mean and superior quadrant peripapillary RNFLT was slightly less in diabetic patients without abnormal vascular manifestations than in healthy subjects. Furthermore, lower RNFLT values in patients with preclinical DR appears to be weakly associated with high levels of fasting blood sugar.