Telomerase is a ribonucleoprotein reverse transcriptase that synthesizes telomeric DNA onto chromosome ends, and is not detected in most normal cells. It has been clarified that some bronchial squamous cell carcinomas may arise through the metaplasia and dysplasia sequence accompanied by accumulation of genetic mutations in metaplastic cells. Recently a highly sensitive polymerase chain reaction (PCR)-based telomerase assay (TRAP assay) was developed for the detection of telomerase activity. Telomerase activity has been found in most malignant neoplasms, including lung cancer. The objective of this study was to determine whether telomerase RNA might increase in precancerous lesions of the bronchi. Bronchial-brushing extracts were analyzed for telomerase activity (F-TRAP) and in situ telomerase activity using a fluorescence-based TRAP assay (in situ TRAP) and compared to cytological features. The fluorescence-based semi-quantitative TRAP assay detected telomerase activity in 8 out of 12 lung cancer cases (66.7%). In squamous cell carcinoma, 6 out of 9 cases (66.7%) showed telomerase activity. On the other hand, in normal and precancerous lesions of the bronchi, telomerase activity was not detected using either the F-TRAP method or in situ TRAP method. We concluded that dysplastic cells might not contain immortalized cells, and that the increase of telomerase activity is a relatively late event during the bronchial carcinogenesis. It is difficult to distinguish between dysplasia and in situ carcinoma of the bronchus morphologically, but the measurement of telomerase activity is clinically valuable for the determination of treatment.