Background: Obstructive sleep apnea syndrome (OSAS) has been shown to generate hypertension and endothelial dysfunction. Retinal vessel is the only vessel that can be observed directly and noninvasively; retinal vascular abnormalities can serve as a predictive marker for the occurrence, clinical course, and prognosis of cardiovascular and cerebrovascular diseases. The objective of this study was to identify the effect of OSAS severity on the morphological changes of retinal vessels. Methods: Adult patients complained of snoring were included in this study. The patients' general information, polysomnography, and fundus photography parameters including central retinal artery equivalent (CRAE), central retinal vein equivalent (CRVE), and arteriole-to-venule ratio (AVR) were collected. Patients were divided into four groups according to their apnea-hypopnea index (AHI) results: Group I, AHI <= 5/h; Group II, 5/h < AHI <= 30/h; Group III, 30/h < AHI <= 60/h; and Group IV, AHI> 60/h. Results: A total of 133 patients were included in this study with 111 males (83.5%) and 22 females (16.5%). Mean age was 41.6 +/- 9.9 years, and the mean body mass index was 28.1 +/- 4.0 kg/m(2). AHI ranged between 0 and 130.8/h with a mean of 39.1 +/- 30.7/h. There were 24, 34, 35, and 40 patients in Group I, Group II, Group III, and Group IV, respectively. Significant differences were found for AHI (F = 388.368, P< 0.001), minimal pulse oxygen saturation (F = 91.902, P< 0.001), and arousal index (F = 31.014, P< 0.001) among four groups; no significant differences were found for CRAE (F = 0.460, P = 0.599) and CRVE (F = 0.404, P = 0.586) among groups; there were significant differences for AVR between Group I and Group IV (63.6 +/- 5.1% vs. 67.2 +/- 5.5%, P = 0.010) Group II and Group IV (64.5 +/- 6.0% vs. 67.2 +/- 5.5%, P = 0.030), and Group III and Group IV (64.7 +/- 4.1% vs. 67.2 +/- 5.5%, P = 0.043). A main group-by-AHI effect was found on the AVR: patients with higher AHI showed higher AVR results (r = 0.225, P = 0.009). Multivariate logistic regression analysis was used for multi-variable factors. A group-by-age effect was found on the AVR: younger patients showed higher AVR results (beta = -0.001, P = 0.020). Conclusions: This study indicated that increased AVR of retinal vessel can be observed in extremely severe OSAS patients. For patients with OSAS, retinal vascular abnormalities may become an early indication for further cardiovascular abnormalities.