Objective To asscess cognitive changes of patients with Obstructive Sleep Apnea-Hypopnea Syndrome(OSAHS)by the Expansive Scale of Dementia(ESD),and to compare ESD to Mini Mental Status Examenation(MMSE)in cognitive function.Methods 90 OSAHS patients were investigated in our study.All patients were divided into three groups,according to apnea-hypopnea index(AHI),including mild group,moderate group and severe group.The cognitive function was evaluated by MMSE and ESD.Results The parameters,including AHI,Longest apnea time(LAT)and percentage of time spend when oxygen saturation lower than 0.9(SLT 90%) in severe OSAHS group were significantly higher than those in mild and moderate groups(P0.05).The total score of MMSE and subtest scores of MMSE in memory,attention,calculation and short-and long-term memory were significantly lower in severe OSAHS group than those in mild and moderate groups(P0.05).Conclusion There was the obvious decrease in cognitive function in patients with severe OSAHS,especially in leaning,memory,calculation and constructive function.Both ESD and MMSE could be used to evaluate the cognitive function in OSAHS patients,especially ESD.