Purpose: In the present study we evaluate the corneal endothelium using specular microscopy in patients with obstructive sleep apnea syndrome (OSAS). Methods: The study included a total of 100 patients including 35 patients with mild OSAS, 34 patients with moderate OSAS and 31 patients with severe OSAS, and the right eyes of 30 patients as a control group. Patients were examined to exclude the possibility of ocular diseases. Cellular density in the cornea epithelium (cell/mm(2)), corneal thickness (mu), percentage of hexagonal cells (%) and the coefficient of variation were evaluated using a specular microscope. Results: Corneal thickness was significantly decreased in all OSAS groups when compared to the control group (p = 0.002), while no significant difference was identified among the OSAS groups. The corneal endothelial cell density, percentage of hexagonal cells and coefficient of variation were significantly different between the OSAS groups and the control group (p < 0.001). Conclusion: More significant impairments were noted in the corneal endothelium of the patients in the OSAS group than in the control group, and specular microscopy is in valuable in the follow-up and treatment of such patients.