Study methods. The study will be a double- blind randomised placebo controlled trial with a parallel group design assessing the addition of 25mg spironolactone to existing antihypertensive treatment, titrated to 50mg if necessary in 40 patients with resistant hypertension (20 patients per group) over a period of 6 weeks. Resistant hypertension will be defined as: blood pressures not adequately controlled (systolic blood pressure (SBP) >140 mmHg and/or diastolic blood pressure (DBP) >85 mmHg in clinic and on ambulatory blood pressure monitoring) despite treatment with the maximum tolerated dose of three antihypertensive agents. All patients' current antihypertensive treatment will include a thiazide diuretic and at least one of a beta-blocker, angiotensin converting enzyme inhibitor or angiotensin II receptor antagonist. The primary endpoint will be the difference between the spironolactone and placebo groups in change in daytime average systolic blood pressure on 24-hour ambulatory blood pressure monitoring from day 0 to day 42.