Telomerase activation has been demonstrated in both cancers and some noncancerous lesions. However, few studies have determined levels of telomerase activity in these lesions. In the present study, using a recently developed stretch PCR assay, telomerase activity was quantitatively determined in a variety of ovarian lesions including 36 ovarian cancers, 5 ovarian low potential malignancy (LPM) lesions, 10 ovarian cysts, and 12 normal ovaries. Telomerase activity was normalized to control activity (100 units) in C33A cell line and given in relative units. Telomerase activity in ovarian cancer (51 +/- 7 units, mean +/- SE) was significantly higher than that in LPM lesions, ovarian cysts, and normal ovaries (7 +/- 3, 10 +/- 2, and 10 +/- 2 units, respectively; P < 0.001). Interestingly, all LPMs, ovarian cysts, and normal ovaries exhibited low telomerase activity less than 30 units, and no significant difference in level of telomerase activity was found among them. We also found a significant correlation between the level of telomerase activity and the clinical stage of ovarian cancer. Our quantitative telomerase assay thus clearly distinguished telomerase activity in ovarian cancers from that in LPM lesions and ovarian cysts. Significant levels of telomerase activation frequently occurred in cancer but rarely occurred in premalignant and benign lesions, suggesting that telomerase activation is a critical step in cancer development.