Purpose <p>Evaluation of scleral thickness with anterior segment optical coherence tomography (AS-OCT) in patients with obstructive sleep apnea syndrome (OSAS).</p> Study design <p>Cross-sectional study.</p> Methods <p>A total of 30 severe (Group 1), 30 moderate (Group 2) and 30 mild (Group 3) OSAS patients and 22 healthy volunteers (Group 4) were included in the study. Scleral thickness measurements were taken with AS-OCT, 6 mm, 4 mm and 2 mm posterior to the scleral spur, in four gaze positions. Data and findings were examined comparatively<b>.</b></p> Results <p>At 6 mm posterior to the scleral spur, the mean thickness was found to be significantly higher in all quadrants in Group 1 (p&lt;0.05 for all). Superior thickness was higher in Group 2 compared to Group 4 (p=0.034). At 4 mm posterior to the scleral spur; in Group 1, the mean thickness was found higher than Group 2 and 3 only in the inferior (p=0.01, p=0.021, respectively) and was found higher than Group 4 in the nasal and inferior (p&lt;0.001, p=0.006, respectively). At 2 mm posterior to the scleral spur; significant difference was observed between Groups 1 and 2 only in the inferior (p=0.002), between Groups 1 and 3 in the nasal and inferior (p=0.029, p=0.002), and between Groups 1 and 4 the in nasal and inferior (p=0.002, p&lt;0.001).</p> Conclusion <p>The scleral thickness measured from 6 mm posterior to the scleral spur was higher in all quadrants in patients with severe OSAS. It is possible that the increase in scleral thickness, especially in severe OSAS, is due to extracellular matrix accumulation in the scleral tissue.</p>