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Background/Aim. It is necessary to find relevant oxidative stress markers for predicting the severity of obstructive sleep apnea-hypopnea syndrome (OSAHS), a sleep disorder-related respiratory disease. The aim of the study was to investigate if there is a correlation between oxidative stress and cognitive impairment in OSAHS patients. Methods. A total of 220 patients were divided into the group of snoring patients, the group with mild to moderate OSAHS, and the group with severe OSAHS according to polysomnography (PSG). Apnea-hypopnea index (AHI), oxygen desaturation index (ODI), and baseline data were monitored. Oxidative stress indices were measured by colorimetry from blood samples taken early in the morning. The patients were then divided into the group with normal cognition and cognitive impairment group based on minimental state examination (MMSE) and Montreal cognitive assessment (MoCA). Independent risk factors for cognitive impairment were analyzed by multi-variate logistic regression. The correlation between oxidative stress and cognitive impairment was analyzed by Pearson’s method. Receiver operating characteristic (ROC) curves made it possible to analyze the efficiency of oxidative stress combined with detection for assessing cognitive impairment in OSAHS patients. Results. The snoring group, mild to moderate OSAHS group, and severe OSAHS group had significantly different snoring loudness, body mass index (BMI), AHI, ODI, MoCA, and MMSE scores, and levels of malondialdehyde (MDA), glutathione peroxidase (GSH-Px), and superoxide dismutase (SOD) (p < 0.05). The cognitive impairment group and group with normal cognition had different BMI, GSH-Px, MDA, SOD, neuroglobin, hypoxia-inducible factor, AHI, and lowest nocturnal oxygen saturation (p < 0.05 or p < 0.01) levels. BMI, GSH-Px, MDA, SOD, neuroglobin, hypoxia-inducible factor, AHI, and lowest nocturnal oxygen saturation were independent risk factors for cognitive impairment. The MoCA and MMSE scores of cognitive impairment had positive correlations with GSH-Px and SOD but negative correlations with MDA (p < 0.05). The area under the ROC curve of GSH-Px, MDA, and SOD and their combination for prediction of cognitive impairment were 0.670, 0.702, 0.705, and 0.836, respectively. Conclusion. Oxidative stress may be the biochemical basis of cognitive impairment in OSAHS patients.