In the treatment of most medical conditions, there are many choices. A critical question for practicing clinicians is: "Are all drugs within a class interchangeable?" In the past decade, the market has seen a proliferation of drugs within popular drug classes. The original drugs within a class typically have better scientific documentation than the newer ones, which are often referred to as "me-too" drugs. Due to a lesser financial investment, the latter may be available at a lower cost. Good reasons exist for grouping drugs, however, there is no accepted definition of the term "class effect." Although members of a drug class share main actions, they may have clinically important differences in terms of efficacy and safety. There are many such examples in the literature. This article reviews the class effect concept as it applies to the angiotensin-converting enzyme (ACE) inhibitors. Only half of the 10 ACE inhibitors available in the U.S. have been shown to improve survival and reduce morbidity in patients with heart failure or myocardial infarction. It is unknown whether the other five have the same safety and efficacy profiles or what their optimal doses are. Thus, we do not know whether all ACE inhibitors are fully interchangeable. The practice of medicine ought to be based on solid scientific evidence, not on assumptions or extrapolations. For our patients, such practice is a legitimate expectation. Therefore, it seems prudent to recommend that patients requiring ACE inhibitor therapy be prescribed one that has been proven effective and safe.