Telomeres fulfill the dual function of protecting eukaryotic chromosomes from illegitimate recombination and degradation and may aid in chromosome attachment to the nuclear membrane. We have previously shown that telomerase, the enzyme which synthesizes telomeric DNA, is not detected in normal somatic cells and that telomeres shorten with replicative age. In cells immortalized in vitro, activation of telomerase apparently stabilizes telomere length, preventing a critical destabilization of chromosomes, and cell proliferation continues even when telomeres are short. In vivo, telomeres of most tumors are shorter than telomeres of control tissues, suggesting an analogous role for the enzyme. To assess the relevance of telomerase and telomere stability in the development and progression of tumors, we have measured enzyme activity and telomere length in metastatic cells of epithelial ovarian carcinoma. We report that extremely short telomeres are maintained in these cells and that tumor cells, but not isogenic nonmalignant cells, express telomerase. Our findings suggest that progression of malignancy is ultimately dependent upon activation of telomerase and that telomerase inhibitors may be effective antitumor drugs.