Obstructive sleep apnea syndrome (OSAS) is strongly associated with cardiovascular disease which means these patients could suffer sudden death from these cardiovascular diseases, but it is a rare case that common OSAS itself causes sudden death directly. On the other hand, we never wrongly diagnose serious sleep-related breathing disorders (SBDs) such as sleep hypoventilation syndrome, OSAS patients with cardiovascular disease (i.e. ischemic heart disease, cardiac arrhythmia and cardiac failure) and bilateral vocal cord paralysis caused by multiple system atrophy as common OSAS. This section describes how to distinguish these SBDs from common OSAS.