Obstructive sleep apnoea (OSA) and hyperaldosteronism are very common in subjects with resistant hypertension. We hypothesized that aldosterone-mediated chronic fluid retention may influence OSA severity in patients with resistant hypertension. We tested this in an open-label evaluation by assessing the changes in the severity of OSA in patients with resistant hypertension after treatment with spironolactone. Subjects with resistant hypertension (clinical blood pressure (BP) ≥140/90 mm Hg on ≥3 antihypertensive medications, including a thiazide diuretic and OSA (defined as an apnoea-hypopnoea index (AHI) ≥15) had full diagnostic, polysomnography before and 8 weeks after spironolactone (25-50 mg a day) was added to their ongoing antihypertensive therapy. In all, 12 patients (mean age 56 years and body mass index 36.8 kg m-2) were evaluated. After treatment with spironolactone, the AHI (39.8±19.5 vs 22.0±6.8 events/h; P<0.05) and hypoxic index (13.6±10.8 vs 6.7±6.6 events/h; P<0.05), weight and clinic and ambulatory BP were significantly reduced. Plasma renin activity (PRA) and serum creatinine were significantly higher. This study provides preliminary evidence that treatment with a mineralocorticoid receptor antagonist substantially reduces the severity of OSA. If confirmed in a randomized assessment, it will support aldosterone-mediated chronic fluid retention as an important mediator of OSA severity in patients with resistant hypertension. © 2010 Macmillan Publishers Limited.