Obstructive sleep apnea (OSA) is commonly associated with systemic hypertension and now recognized as an independent risk factor for daytime hypertension. We aimed to study the short- and long-term effect of nasal continuous positive airway pressure (CPAP) in hypertensive and normotensive patients with OSA. Forty-six patients with moderated to severe OSA were treated with nasal CPAP and followed after one year of treatment. Clinic blood pressure, heart rate, and body weight were taken before and followed up for one year after beginning nasal CPAP. In this study 25 patients with OSA were found to have hypertension (54.3%). The hypertensive group showed a significant reduction in clinic blood pressure after nasal CPAP, whereas the normotensive group showed no changes. The subgroup of hypertensive patients with OSA who had no anti-hypertensive medication revealed a decrease in clinic blood pressure comparable to those with anti-hypertensive drugs. The heart rate was not significantly changed in any patients. There was no significant correlation between the decrease in body weight and the reduction in blood pressure. These results suggest that nasal CPAP alone might have a substantial blood pressure lowering effect in hypertensive patients with OSA. This effect could decrease the morbidity and mortality related to cardiovascular complications in patients with OSA.