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In newly diagnosed apnoeic patients without a history of hypertension, clinical hypertension is underdiagnosed in at least 40% of the cases. An increase in diastolic blood pressure is the most frequent pattern encountered.To assess clinic and 24-h blood pressure, baroreflex sensitivity and left ventricular mass for identifying the prevalence, the type and the consequences of hypertension in newly diagnosed apnoeic patients.Fifty-nine unselected patients (age = 48 ± 12 years, body mass index = 28.1 ± 4.5 kg/m2) referred to a university hospital sleep laboratory for symptoms suggesting obstructive sleep apnoea were included. Diagnosis of apnoea was accepted when respiratory disturbance index was ≥15/h of sleep. Blood pressure was considered as normal by their general practitioner and all of them were free of any medication for hypertension. Echocardiography, 24-hour ambulatory blood pressure monitoring and assessment of the baroreflex sensitivity were performed.Forty-two percent of the apnoeic patients demonstrated a clinical hypertension, 34 subjects (58%) had a daytime hypertension, and 45 patients (76%) had a night-time hypertension, using 24-h monitoring. All the daytime hypertensives also had night-time hypertension. Forty-seven of the 59 patients (80%) were hypertensive either in the clinic or using 24-h recording. Diastolic and systolo-diastolic hypertension were the prominent types of hypertension observed both by clinic or ambulatory measurements. Respiratory disturbance index was significantly higher in apnoeic patients suffering isolated diastolic hypertension than in the normotensives (50.9 ± 26.5/h versus 36.0 ± 12.3/h, respectively; P = 0.02). The prevalence rate of left ventricular hypertrophy was high (between 15 and 20%) and occurred independently of associated hypertension. Baroreflex sensitivity was altered whatever the type of hypertension and decreased with the severity of obstructive sleep apnoea.Hypertension is hugely underdiagnosed in apnoeic patients unknown to be hypertensive. Use of 24-h blood pressure monitoring allowed the diagnosis of twice as much hypertension than did clinical measurement. Even at the beginning of their history of hypertension, apnoeic patients exhibited chronic adaptations of their cardiovascular system, as shown by early changes in baroreflex sensitivity and an increased prevalence of left ventricular hypertrophy.