Since neither angiotensin-converting enzyme inhibitors (ACE-I) nor angiotensin II receptor blockers (ARB) can completely suppress aldosterone levels, there is a need for alternative/supplementary antihypertensive medications, such as the selective aldosterone blocker eplerenone (Inspra). This multicenter study measured the safety and efficacy of add-on eplerenone therapy to reduce blood pressure not controlled by ACE-I or ARB monotherapy. An ad hoc analysis evaluated whether active plasma renin or serum aldosterone levels could predict blood pressure response to eplerenone therapy. Patients (N = 341) with a diastolic blood pressure > 95 mmHg on a fixed dose of ACE-I or ARB were randomized to 8 weeks of double-blind treatment with eplerenone 50 mg qd or placebo. If blood pressure remained uncontrolled following 2, 4, or 6 weeks of treatment, the eplerenone dose was increased to 100 mg qd. In a combined cohort analysis of these patients, the placebo-adjusted change in systolic and diastolic blood pressure was -5.9/-2.4 mmHg (p< 0.001 and p = 0.006, respectively). While adding eplerenone to an ACE-I or ARB is safe and effective for blood pressure reduction, there was no baseline value or range of values of active plasma renin, serum aldosterone, or their ratio that predicted a favorable response to either of these drug combinations.