BACKGROUNDAmbulatory blood pressure (BP) monitoring (ABPM) detects subjects with normal clinic but high ambulatory 24-h BP, that is, masked hypertension. METHODSOne hundred and thirty newly diagnosed obstructive sleep apnea syndrome (OSAS) patients, free of recognized cardiovascular disease were included (111 men, age = 48 ± 1 years, BMI = 27.6 ± 0.4 kg/m, respiratory disturbance index (RDI = 42 ± 2/h). Clinic BP, 24-h ABPM, baroreflex sensitivity (BRS), echocardiography and carotid intima–media thickness (IMT) were assessed. RESULTSForty-one patients (31.5%) were normotensive, 39 (30.0%) exhibited masked hypertension, four (3.1%) white-coat hypertension and 46 (35.4%) hypertension. Significant differences were found between normotensive, masked hypertensive and hypertensive patients in terms of BRS (10.5 ± 0.8, 8.0 ± 0.6 and 7.4 ± 0.4 ms/mmHg, respectively, P < 0.001), carotid IMT (624 ± 17, 650 ± 20 and 705 ± 23 μm, respectively, P = 0.04) and left ventricular mass index (37 ± 1, 40 ± 2 and 43 ± 1 g/height, respectively, P = 0.003). A clinic systolic BP more than 125 and a diastolic BP more than 83 mmHg led to a relative risk (RR) of 2.7 and a 90% positive predictive value for having masked hypertension. CONCLUSIONMasked hypertension is frequently underestimated in OSAS and is nearly always present when clinic BP is above 125/83 mmHg.