Captopril is an orally active inhibitor of angiotensin-converting enzyme (ACE) and has been widely studied in the treatment of patients with mild to moderate essential hypertension, severe hypertension not responsive to conventional diuretic/beta-adrenoceptor blocker/vasodilator regimens, and patients with chronic congestive heart failure refractory to treatment with a diuretic and digitalis. In patients with mild or moderate essential hypertension, titrated low doses of captopril used alone or in conjunction with a diuretic are similar in efficacy to usual doses of hydrochlorothiazide, chlorthalidone, or beta-adrenoceptor blocking drugs, as well as to the other ACE inhibitors. In addition, captopril improved well-being to a greater extent than methyldopa or propranolol in a study designed specifically to determine the effect of treatment on the quality of life of patients with mild or moderate essential hypertension. The earlier demonstrated efficacy of captopril, used with a diuretic and often also with a beta-adrenoceptor blocking drug, in the treatment of severe hypertension refractory to conventional 'triple therapy' has been confirmed in more recent trials which illustrate the generally marked antihypertensive effect of captopril-containing regimens in such patients. Results of initial trials in patients with scleroderma are promising, with control of hypertension and stabilization of renal function in these patients when treated at an early stage of the disease. Several comparative and long term trials of captopril in patients with chronic congestive heart failure refractory to treatment with a diuretic/digitalis regimen clearly demonstrate that initial haemodynamic improvement is maintained and correlates with clinical benefit. A tendency for overall clinical response to captopril to be better than the response to prazosin, hydralazine, nisoldipine or enalapril has been reported. Results of a multicentre comparison with digoxin and placebo indicate that captopril is a suitable alternative to digoxin in patients with mild to moderate heart failure who are receiving maintenance diuretic therapy. The tolerability of captopril has now been studied in many thousands of patients involved in formalized trials and the early impression of poor tolerability can no longer be justified. The use of generally lower dosages of captopril in patients with normal or slightly impaired renal function has resulted in a generally low incidence of rash (0.5 to 4%), dysgeusia (0.1 to 3%), proteinuria (0.5%), neutropenia (0.3% during first 3 months) and symptomatic hypotension (0.1 to 3%). Cough is an infrequent but troublesome effect resulting from ACE inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)