BACKGROUND: Telomerase enzyme activity is not detected in most normal cells, a phonomenon believed to be associated with limitations on cellular proliferation. Since this activity is detected in nearly all human tumor, including lung cancers, it has been suggested that telomerase activation may be coupled to acquisition of malignant phenotype. In this study, we determined whether telomerase activity was associated with tumor pathologic stage. METHODS: Primary tumor specimens obtained by bronchoscopic biopsies from 33 patients were analyzed. Telomerase activity was measured by means of a modified Telomeric Repeat Amplication Protocol(TRAP) assay. RESULTS: Telomerase activity was detected in 23 of the 27 non small cell lung cancer and 5 of 6 small cell lung cancer. A few primary tumors did not appear to have detectable telomerase activity. Positive associations were found between the telomerase-positive rate and tumor stage(p<0.05). CONCLUSION: High telomerase activity is detected frequently in primary lung cancers that exhibit high tumor cell proliferation rates and advanced pathologic stage.