Heart failure is difficult to define and quantify, and this makes it difficult to assess the relative value of different drug regimens. Although there are theoretical reasons suggesting that ACE inhibitors might be useful in all patients with heart failure, there are cogent arguments to the contrary. In practice, angiotensin-converting enzyme inhibitors have been shown to give considerable relief to patients with severe heart failure, but in patients who are only moderately ill these drugs may have less effect than increasing the dose of diuretics. Vasodilators in general probably reduce the fatality of patients with heart failure, but the effect is not specific to angiotensin-converting enzyme inhibitors.