Must great harm often follow as part of the natural course of progress in medicine? I believe the answer. . . is no. While it is unrealistic to believe we can formulate any principles for determining with certainty what will prove therapeutically effective, we can develop ways of limiting the harm done until the answer is obtained. In my view, it is essential that we minimize the harm caused by premature claims of cures, by unbridled ambition, and by uncritically enthusiastic promotion. This end can be accomplished only by establishing procedures for testing innovative therapies before they are broadly used.