Scientific data about the effects of positive airway pressure (PAP) treatment on blood pressure (BP) control are continuously increasing; however, they are controversial. We aimed to determine the long-term effects of compliance with PAP therapy on BP in both hypertensive and normotensive patients with obstructive sleep apnea-hypopnea syndrome (OSAHS). One thousand one hundred sixty eight consecutive patients with newly diagnosed OSAHS, who had been recommended PAP therapy, were followed up for a minimum of 2 years. Patients with previous cardiovascular disease were excluded. BP was measured at baseline and after 2 years of PAP treatment. In addition, the correlation between the changes in BP with different levels of PAP compliance was assessed. At the end of the follow-up period, in the hypertensive group of patients (n=586), a significant decrease was shown in systolic (-11.2 mm Hg, P<0.001) and diastolic BP (-4.2 mm Hg, P<0.001). Furthermore, in the patients without hypertension (n=528), a significant decrease was noted both in systolic and diastolic BP (-3.6, P<0.001 and -2.4, P<0.001, respectively). A correlation between the magnitude of change in systolic and diastolic BP and hours of use of PAP (r=0.14, P=0.002 and r=0.1, P=0.025, respectively) was observed in all patients. Long-term use of PAP treatment, as well as increased hours of PAP in patients with OSAHS use showed significant reductions in BP not only in patients with hypertension, but also in normotensive patients. Therefore a significant potential reduction in cardiovascular mortality and morbidity should be expected in these patients.