AIM: To assess choroidal thickness in patients with severe obstructive sleep apnea syndrome (OSAS) and compare them with healthy controls, using spectral domain optical coherence tomography (OCT).METHODS: In this observational, cross-sectional study, choroidal thicknesses of 23 newly severe OSAS patients and 23 body mass index- age- and sex-matched healthy subjects were measured using a high-speed, high-resolution frequency domain-OCT device (λ=840 nm, 26000 A-scans/s, 5 μm axial resolution). All patients underwent a complete ophthalmic examination before the measurements. OCT measurements were taken at the same time of day (9:00 a.m.), in order to minimize the effects of diurnal variation.CONCLUSION: The decreased choroidal thickness of patients with severe OSAS might be related to the the autonomic disregulation associated with this disease. Further studies are needed to evaluate the etiopathologic relationship between choroidal thickness and OSAS.