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Obstructive sleep apnea syndrome (OSAS) is closely associated with hypertension. Activity of angiotensin II (Ang II) and non-dipping nocturnal blood pressure (BP) variability are implicated in hypertension-related target organ damage. We examined the correlation between OSAS with serum Ang II levels and evaluated the risk of non-dipping BP variability in 180 patients with essential hypertension (EHT). Eligible patients were divided into three subgroups based on their apnea-hypopnea index (AHI) evaluated by polysomnography. EHT alone, EHT with mild OSAS, and EHT with moderate/severe OSAS. Ambulatory BP monitoring was used to calculate mean BP over 24 h, as well as diurnal and nocturnal BP variability. Serum Ang II was determined with enzyme-linked immunosorbent assay. EHT patients with OSAS had significantly higher systolic BP calculated either over 24 h, or by diurnal or nocturnal monitoring ( P < 0.05). More EHT patients with OSAS showed non-dipping BP profiles than did EHT patients alone ( P < 0.05). The number of patients with non-dipping BP increased with increasing OSAS severity. Surgical treatment alleviated OSAS and reduced AHI ( P < 0.05). Preoperative serum Ang II in EHT patients with OSAS was significantly higher than that in those without OSAS ( P < 0.05), and showed a rising trend with OSAS severity ( P < 0.05). Postoperative serum Ang II, BP and the incidence of non-dipping BP were reduced by surgery to levels lower than preoperative values in patients with OSAS. We therefore conclude that OSAS leads to increased serum Ang II and increased risk of non-dipping BP in patients with EHT. [ABSTRACT FROM AUTHOR]