Obstructive sleep apnea (OSA) is found to affect 2-4% of the middle-aged population in several Caucasian studies, whereas the prevalence among other ethnic groups have not been clearly documented. It has been reported that OSA and systemic hypertension are highly associated; we therefore conducted a study on Chinese subjects who were receiving treatment for essential hypertension to assess the prevalence of OSA among this group. Ninety-two consecutive patients being followed up at a hypertension clinic were recruited for a questionnaire survey. The entire study group was aged 54.7 +/- 11.7 years, with 40 men. One male subject had a diagnosis of obstructive sleep apnea on nasal continuous positive airway pressure (nCPAP) treatment and 46 subjects agreed to an overnight sleep study. Those who underwent sleep study showed selection bias with a higher body mass index and more symptoms associated with OSA. Of the 46 who underwent sleep study, 16 (34.8%) had an obstructive apnea-hypopnea (AHI) score of >/=5 and excessive daytime sleepiness, with a median score of 26.2 (range, 8.3-64.9). Patients in the group with obstructive sleep apnea syndrome (OSAS) thus defined compared with those without OSAS had more men (64.7 vs 17.20%, p = 0.001) and an excess of smokers (31.5 vs 3.3%. p = 0.01) and had significantly more symptoms of excessive daytime sleepiness (p = 0.001), daytime fatigue (p = 0.007), and witnessed apneas (p = 0.008). Seven patients accepted treatment with nCPAP and reported improvement in symptoms, but there was no detectable change in clinic blood pressure measurements after 3 months of nCPAP treatment. This study demonstrated a high prevalence of previously unidentified OSAS among Chinese patients with essential hypertension. Increased awareness of both doctors and patients toward this potentially treatable problem is warranted.