목적: Diagnosis and severity of the obstructive sleep apnea(OSA) is usually decided with apnea-hypepnea index (AHI) score. Patients are usually classified as mild (5-15), moderate (15-30) and severe (more than 30) according to AHI. When scoring AHI, several criteria to define hypopnea have been presented. Therefore, severity of OSA may be different in different criteria. The aim of the study is to compare the AHI score according to different criteria and find a difference according to relationship with other parameters. 방법:Patients who admitted to Kyung hee University Medical Center Sleep Clinic and underwent PSG from august 2012 to february 2014 were included in this study. As basis of hypopnea, standard rule (30% airflow reduction and 4% desaturation or arousal) were defined as AHI1, and AHI1 score plus alternative rule (50% airflow reduction and 3% desaturation or arousal) were defined as AHI2. Change of severity classification according to AHI1 and AHI2 and relationship of AHI1 and AHI2 with O2 saturation parameters and respiratory parameters were investigated. 결과:AHI2 score was not statistically different from AHI1 score in all severity groups. However, the classification of severity by AHI showed difference between AHI1 and AHI2 score. In mild OSA of AHI1 scoring, 40.37% of patients changed to moderate OSA. And 27.37% of moderate OSA in AHI1 scoring became severe OSA in AHI2 scoring. Depending om the aging, gap between AHI1 and AHI2 showed gradually increasing pattern but, that was not statistically significant. According to relationship with O2 saturation parameters and RMI, there was no significant difference between AHI1 and AHI2. AHI1 and AHI2 scores are in parallel with all other parameters. 결론:Even though there is no meaningful difference between AHI1 and AHI2 scores, the classification of severity can be changed in significant number of patients. Therefore, we have to be careful when we need to change the criteria in sleep laboratory.