Continuous angiotensin-converting enzyme (ACE) inhibitor therapy does not necessarily produce significant decreases in plasma aldosterone levels (aldosterone escape). We examined the role of aldosterone escape in 75 essential hypertensive patients treated with an ACE inhibitor (enalapril maleate [34 patients], imidapril hydrochloride [24 patients] or trandolapril [17 patients]) for 40 weeks. With treatment, blood pressure decreased and plasma renin activity increased, while plasma aldosterone concentrations did not change. Aldosterone escape was observed in 38 of the 75 patients and in 17 of 37 patients with left ventricular hypertrophy before treatment. Left ventricular mass index did not change in patients with aldosterone escape but decreased significantly in patients without aldosterone escape. The present study demonstrated a high incidence of aldosterone escape in patients with essential hypertension despite the use of ACE inhibitors. The results also suggest that aldosterone escape may reverse the beneficial effects of an ACE inhibitor on left ventricular hypertrophy.