Obstructive sleep apnea syndrome (OSAS) is a common disorder with prevalence rates of at least 4% among the middle-aged male Caucasians and Hong Kong (HK) Chinese populations. It is characterized by repetitive episodes of upper airway obstruction, causing intermittent hypoxia, sleep fragmentation, disabling daytime sleepiness, impaired cognitive function and poor health status. OSAS patients are at increased risks of non-fatal and fatal cardiovascular morbidity and mortality including sudden death, in addition to being more prone to road traffic accidents. However, there is no long term follow up of patients with OSAS on the incidence of cardiovascular disease, compliance on the therapy and the impact of the quality of life. The study is aimed to carry out a prospective observational study assessing the role of OSAS as a possible cause of increased cardiovascular risk and cognitive decline.