Cilazapril is a new once-daily angiotensin-converting (ACE) enzyme inhibitor which has been administered to 4,500 patients with mainly mild to moderate essential hypertension in a multinational clinical research program. Sitting diastolic blood pressure was reduced by about 9 mm Hg from baseline (p < 0.01) after 4 weeks of treatment with cilazapril 1.25-10 mg/day in double-blind placebo-controlled studies. Total responder rates to cilazapril were usually 50-60% compared with 30% to placebo. Adding hydrochlorothiazide 12.5 mg/day to cilazapril 5.0 mg/day increased the total responder rate from 52 to 71 %. Double-blind dose titration studies for 8 weeks showed that cilazapril 2.5-5 mg/day possessed equivalent efficacy to usual therapeutic regimens of sustained release pro-pranolol, captopril, hydrochlorothiazide, atenolol and enalapril. Cilazapril did not affect heart rate. During long-term open administration for 52 weeks, or longer, cilazapril, either alone or in combination with hydrochlorothiazide, effectively maintained control of blood pressure. Treatment of patients with severe hypertension with cilazapril plus hydrochlorothiazide achieved a total responder rate of 73 %. Adverse events were mostly observed within the first 8-16 weeks of treatment, with headache, dizziness, fatigue, nausea, cough and chest pain being the most frequent. Non-life-threatening angioedema, facial edema and mild hypotension occurred in ≤ 0.2% of patients, and orthostatic hypotension was reported in 2%. Abnormal laboratory test values were rarely found with cilazapril treatment. Of the 2.3% of patients with elevated serum creatinine, at any time point during the study and irrespective of outcome on continuation with cilazapril therapy, about two thirds had prior renal impairment. This was also the situation in 5 of 23 patients (0.6%) with proteinuria. Overall, < 6 % of patients withdrew from treatment because of adverse events. In conclusion, the results show that cilazapril possesses good efficacy and excellent tolerability in the treatment of essential hypertension.