Obstructive sleep apnoea (OSA) is a highly prevalent sleep disorder; it affects 4% of males and 2% of females. Hypertension has been shown to occur in 28-57% of OSA patients. There is a steady increase in evidence linking OSA to long-term cardiovascular morbidity including hypertension. The purpose of this study was to investigate whether mandibular advancement oral appliance (OA) treatment of OSA affects the patient's blood pressure (BP) in a 3-month and a 3-year perspective. Twenty-nine consecutive patients, with verified OSA defined as apnoea index (AI) >5 per hour and/or apnoea/hypopnoea index (AHI) > or =10 per hour, received an OA as treatment. BP was measured on three occasions; before treatment, after 3 months of treatment, and after 3 years of treatment. BP was measured with an electronic blood pressure monitor. The treatment effect of OA was measured after 3 months by repeated somnographic registration while the patient was wearing the OA. A treatment response was defined as AHI < 10; this was achieved in 25 of 29 patients (86%) at the 3-month evaluation. Significant reductions in blood pressure were attained between baseline and the 3-month evaluation (P < 0.001) and these changes remained at the 3-year follow-up in both systolic BP of -15.4 +/- 18.7 mm Hg and diastolic BP of -10.3 +/- 10.0 mm Hg. OA therapy reduced blood pressure in both a 3-month and a 3-year perspective in patients with OSA.