INTRODUCTION: Determination of clinical obstructive sleep apnea syndrome (OSAS) types is important for treatment decision. In the literature, there are two different criteria for the diagnosis of position and rapid eye movement (REM) related OSAS. One of them provides the criteria that nonsupine and/or non-REM apnea-hypopnea index (AHI) below 5, the other does not. In this study, these two definitions are named as "strict definition" and "loose definition." This study is designed to identify which definition is more beneficial to use, and the prognostic value of the definitions by using OSAS severity according to AHI. This is the first study which investigates those issues.