Telomerase activity was measured in biopsy specimens as well as surgically resected tissues of 39 oral squamous cell carcinomas (SCCs), 22 oral leukoplakias, 13 normal oral mucosas, 12 malignant salivary gland tumors, 10 benign salivary tumors and five normal salivary gland tissues adjacent to tumors using a polymerase chain reaction (PCR)-based telomerase assay. Telomerase activity was detectable in 38.5% (5/13) of normal oral mucosa samples, 54.5% (12/22) of leukoplakia samples, 82.1% (32/39) of oral SCC samples, 83.3% (10/12) of malignant salivary gland tumor samples and 0% (0/10, 0/5) of benign salivary gland tumor and normal salivary gland samples. High-level enzyme activities were seen in 20% (3/15) of mild dysplasia specimens, 50% (2/4) of moderate-severe dysplasia specimens, 48.7% (19/39) of oral SCC tissue specimens, and no high activity was seen in the normal mucosa and hyperkeratosis specimens (P for trend, <0.001). We also analyzed the proliferative activity of dysplastic leukoplakia, oral SCC, and salivary gland tumor specimens using Ki-67 immunohistochemistry. The Ki-67 labeling indices (LI) were significantly higher in dysplastic leukoplakia and oral SCC with high telomerase activity than in dysplastic leukoplakias and oral SCC with low and negative telomerase activity (P<0.01 and P<0.05). These results indicate that telomerase activity has some correlation with the progression of multistep oral carcinogenesis with the cellular proliferation, and also indicate that telomerase may be a specific marker used to distinguish malignant salivary gland tumors from their benign counterparts.