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Background and objective: While recent meta-analyses suggest that OSA elevates daytime blood pressure (BP), most studies have included patients with mild hypertension, so CPAP treatment has only reduced BP by 2–3 mm Hg. To determine the strength of the OSA–BP relationship, this study examined the effect of CPAP in a cohort where severe OSA and under-treated hypertension coexist. Methods: Baseline demographic and sleep study data were recorded in 221 consecutive patients referred for sleep studies in Samoa. OSA was treated with CPAP and BP recorded for 6–7 months in 180 patients. A subgroup of 64 patients, representative of the entire group, but with regular follow-up, is also described. Results: OSA was diagnosed in 218/221 patients; of those who commenced CPAP, 180 returned for follow-up at least once within 7 months. Following CPAP, BP decreased by 7.1/5.9 mm Hg at 1 month and 12.9/10.5 mm Hg at 6 months ( n = 180, P < 0.0001). In the 64 patients with regular follow-up, those with the highest baseline BP had the greatest fall in BP with CPAP; BP in the hypertensive subgroup (32/64) decreased 21.5/13.1 mm Hg at 6 months ( P < 0.0001). Conclusions: Hypertensive OSA patients can exhibit large falls in BP with CPAP at 1 month, with further significant reductions at 3 and 6 months. Overall, the fall in BP was proportional to the initial elevation of the BP with many patients achieving normal BP at 6 months. [ABSTRACT FROM AUTHOR]