Epidemiologic studies have shown obstructive sleep apnea (OSA) is an independent risk factor for systemic hypertension. The prevalence of systemic hypertension also increases gradually with age. The purpose of this study was to assess the combined effect of OSA and age on daytime blood pressure. Patients who received nocturnal polysomnography in Tainan Municipal Hospital were invited between October 2008 and February 2010. Daytime blood pressure was measured. Participants were classified into three groups: nonapnea (n = 14, 18%) with RDI <5 episodes/h; mild to moderate OSA (n = 34, 43%) with RDI ≧ 5 and <30; and severe OSA (n = 31, 39%) with RDI ≧ 30. Seventy-nine patients (79/101, 78.2%) (63 males) completed the study. The mean of age, severity of OSA (RDI) and systolic blood pressure (SBP) was 40.3 ± 15.4 years, 28.1 ± 26.0/h and 132.6 ± 19.7 mmHg, respectively. RDI and age were significant risk factors for SBP (P < 0.05). SBP became severe when patients were older in the group of mild to moderate OSA (p = 0.0067) and diastolic blood pressure (DBP) became severe when patients were older in the group of nonapnea and mild to moderate OSA (P = 0.0042 and 0.0168, respectively). But the daytime blood pressure and age were not correlated significantly for the severe OSA subjects. This study revealed that age and RDI were risk factors in development of daytime hypertension. For patients with mild to moderate OSA, SBP was significantly worse when getting older and for patients with nonapnea and mild to moderate OSA, DBP was significantly worse with increasing age.