"TEST DRUGS SHOWN to Eradicate Cancer" proclaimed a headline on May 3, 1998. The first paragraph of the story beneath claimed that two new drugs could "eradicate any type of cancer, with no obvious side effects and no drug resistance." But then came the catch: "in mice."

Many scientists—and ordinary people, too, especially people with cancer and their loved ones—are thrilled by headlines like this, which appear every few years. Many others, however, sigh and say that they have seen it all before, and little or nothing changed as a result. Both groups are right: There is good reason for excitement about drugs like the ones described in the news story, but many earlier treatments greeted with equal excitement have proved to be failures when tested further.

Breakthrough or false hope"

Drugs like the one in...