The progression of chemically induced mouse skin papillomas is paralleled by an increase in telomerase activity. In this study, we compared telomerase activity and rate of cell proliferation in papillomas obtained early versus late in papilloma progression. Eighteen early papillomas (after 15 wk of promotion) showed no evidence of telomerase activity, and their average cell proliferation index was 26.6% ± 6.3. On the other hand, most of the papillomas harvested after 25 wk of promotion showed high levels of telomerase activity, but their average cell proliferation index (30.8% ± 6.2) was not different from that of the early lesions. We concluded that there appears to be no association between the level of telomerase activity observed in mouse skin papillomas and the rate of cell proliferation of each individual tumor. Telomerase expression may indicate the existence of more abundant tumor subpopulations in advanced papillomas with proliferative potential for autonomous growth. Mol. Carcinog. 20:329–331, 1997. © 1997 Wiley-Liss, Inc.