Obstructive sleep apnoea (OSA) is a sleep disorder characterized by recurrent episodes of oxygen desaturation during sleep, representing an independent risk factor for cardiovascular disease, such as myocardial infarction, stroke, congestive heart failure and resistant hypertension. Several neurohormonal mechanisms have been suggested to account for blood pressure increases, such as sympathetic nervous system hyperactivity, oxidative stress, renin-angiotensin-aldosterone system (RAAS) activation, endothelin system activation, and endothelial dysfunction. The aim of this study was to evaluate the behaviour of RAAS and the presence of primary aldosteronism (PA) in these patients and possible correlations between RAAS and the severity of OSA. From October 2007 to November 2008 we studied 325 consecutive newly diagnosed hypertensive patients; 71 patients (21.8%) presented with clinical signs of sleep disorders, evaluated also through a specific questionnaire (Epworth Sleepiness Scale). In hypertensive patients with sleep disorders, 53 patients were affected by OSA; in this group 18 patients were affected by PA (five with aldosterone-producing adenoma (APA) and 13 with bilateral hyperplasia (IHA)); obesity was also demonstrated (BMI > 30 kg/m(2)). Overall, in patients with OSA PRA levels correlated positively with apnoea/hypopnoea index (AHI; r = 0.35; p<0.01), and in all groups the waist circumference and the neck circumference were correlated positively with AHI (r = 0.3 p<0.02 and r = 0.3 p<0.03, respectively). We revealed a high prevalence of PA in patients with OSA, and we can conclude that patients with hypertension and OSA, especially those who are newly diagnosed, must be evaluated for PA.