Background Epidemiologic studies have shown an independent and definite association between obstructive sleep apnea (OSA) and hypertension. This study aimed to define the association between daytime blood pressure and severity of OSA in Chinese population in mainland of China. Methods Twenty university hospital sleep centers in mainland of China were invited by the Chinese Medical Association (CMA) to participate in this epidemiologic study and 2297 consecutive patients (aged 18-85 years; 1981 males and 316 females) referred to these twenty sleep centers for evaluation of OSA between January 2004 and April 2006 were prospectively enrolled. Nocturnal polysomnography was performed in each patient, and disease severity was assessed based on the apnea-hypopnea index (AHI). These patients were classfied into four groups: non-apneic control (control, n=257) with AHI <= 55 episodes/hour; mild sleep apnea (mild, n=402) with AHI >5 and 515 episodes/hour; moderate sleep apnea (moderate, n=460) with AHI >15 and <= 30 episodes/hour and severe sleep apnea (severe, n=1178) with AHI >30 episodes/hour. Daytime blood pressure measurements were performed under standardized conditions in each patient at 10 a.m. in office on the day of referring to sleep centers for getting average value. All the patients were requested to quit medications related to blood pressure for three days before the day of assessing. Results Both daytime systolic blood pressure and diastolic blood pressure values were significantly related to AHI positively (r = 0.201 and 0.276, respectively; both P values <0.001) and to nadir nocturnal oxygen saturation negatively (r = -0.215 and -0.277, respectively; both P values <0.001) which were the parameters of OSA severity. In two special designed mean plots, means of daytime systolic and diastolic blood pressure increased gradually with increasing AHI. Beyond AHI of 61-65, this increasing trend reached a plateau. Conclusions The results showed that OSA severity was associated with daytime blood pressure until AHI of 61-65, providing evidence for early OSA management, especially in OSA patients with concomitant hypertension. Chin Med J 2010;123(1):18-22