Angiotensin-converting enzyme (ACE) inhibitors are clearly effective treatment for all stages of hypertension. Since the introduction of captopril in 1981, numerous ACE inhibitors have been synthesized and are under investigation. Their exact antihypertensive mechanism of action remains unclear. Part of their effect may be mediated by vasodilator prostaglandins. Early studies with as much as 1,000 mg a day captopril demonstrated the agent's ability to reduce blood pressure, but only 10 percent of the severely hypertensive patients were controlled with monotherapy. Subsequent studies have demonstrated that patients with mild to moderate hypertension can be controlled with ACE inhibitor alone, although there is a tendency for the very low doses to lose their effect with time. Black patients are less readily controlled with monotherapy. Captopril has now been demonstrated to be effective in the hypertension of scleroderma and has reversed scleroderma renal crisis. ACE inhibitors are also effective for the treatment of severe congestive heart failure.