The aim of this prospective, randomised, open-label, blinded-end point study was to compare the efficacy and safety of eplerenone versus spironolactone in patients with bilateral idiopathic hyperaldosteronism (IHA). After a 2-week washout period, 34 patients with IHA were assigned to receive either spironolactone 25 mg b.i.d. (n = 17) or eplerenone 25 mg b.i.d. (n = 17) for 24 weeks. If the patients' blood pressure (BP) was not < 140/90 mmHg, the doses were gradually increased up to 400 mg for spironolactone and 200 mg for eplerenone. If the patients' BP remained uncontrolled, a daily dose of hydrochlorothiazide 12.5 mg was added at week 16. The primary outcome was the percentage of patients with BP < 140/90 mmHg at 16 weeks (i.e., with aldosterone antagonist monotherapy). The patients' BP was normalised in 13 out of 17 (76.5%) and 14 out of 17 (82.4%) patients in the spironolactone and eplerenone groups, respectively (p = 1.00). Systolic BP decreased more rapidly with eplerenone. Serum potassium levels were normalised (> 3.5 mmol/l) in all patients at 4 weeks. Mild hyperkalaemia was observed in two patients receiving 400 mg of spironolactone and in three patients receiving 150 mg of eplerenone. Two patients presented with bilateral painful gynaecomastia at the end of week 16 while receiving 400 mg of spironolactone. Switching spironolactone to 150 mg of eplerenone daily resulted in resolution of gynaecomastia and also maintained BP control. At the end of the study, 19 patients were on eplerenone and 15 were on spironolactone. However, this did not affect the primary end point, because the switch from spironolactone to eplerenone (in two patients) occurred at the end of week 16. It was concluded that eplerenone was as effective as spironolactone in reducing BP in patients with IHA. The risk of mild hyperkalaemia was similar with both drugs.