Resistant hypertension is common in clinical practice. The aim of our study was to evaluate inappropriate aldosterone activity in causing resistance to antihypertensive therapy.Among the patients consecutively evaluated for the first time between 1995 and 2001, we selected all those (n = 157) with an aldosterone-to-renin ratio (ARR) >or=25 (ng/dL)/(ng/mL/h), and plasma aldosterone >or=12 ng/dL. Eight patients with Conn adenoma were excluded from the study. Fifty-eight were diagnosed as idiopathic aldosteronism (IHA), the other 91 patients, who did not meet the criteria for primary aldosteronism, were operatively classified as aldosterone-associated hypertension (AAH). As a control group, we randomly chose 160 patients with essential hypertension and plasma aldosterone <12 ng/dL (EH). Antihypertensive treatment was given in accordance to World Health Organization Guidelines (1999). The study end point was blood pressure (BP) <140/90 mm Hg.During follow-up (22 +/- 2 months), 24 (41.4%) patients with IHA, 35 (38.5%) with AAH, and 72 (54.0%) with EH reached the end point. According to survival analysis, AAH and IHA patients reached the end point in a smaller fraction and in a longer time compared with EH patients, with no difference between IHA and AAH. At the end of follow-up, IHA and AAH patients had higher diastolic BP than EH patients with no difference between IHA and AAH.Patients with elevated aldosterone plasma levels develop resistant hypertension, even in the absence of clinically diagnosed primary aldosteronism. Their identification will allow a targeted therapy and a more effective BP reduction.