Animal and epidemiological studies reveal that consuming food and beverages rich in polyphenols (e.g., catechins, flavones, and antocyanines) is associated with a lower incidence of cancer, and several molecular mechanisms have been proposed for explaining this effect. However, because most of these mechanisms were observed only under specific and nonphysiological conditions, and in most cases, with practically irrelevant concentrations, there is still no clear-cut or universal explanation for the major events that underlie the anticancer effects of polyphenols. In this study we present clear in vitro and in vivo evidence that the inhibition of the cancer-associated enzyme telomerase is a key mechanism involved in cancer inhibition by epigallocatechin gallate (EGCG), a major tea polyphenol. We demonstrate that EGCG and other selected polyphenols undergo structural rearrangements at physiologically permissible conditions that result in remarkably increased telomerase inhibition. In nude mice models bearing both telomerase-dependent and -independent xenograft tumors cloned from a single human cancer progeny, only the telomerase-dependent tumors responded to prolonged oral administration of EGCG. Thus, EGCG and likely other structurally related dietary polyphenols seem to act as prodrug-like molecules that, once ingested and distributed, undergo structural changes that favor potent activity against telomerase.