A series of 90 excised cutaneous warts (verrucae vulgaris) were studied for the presence of HPV (human papillomavirus) DNA using in situ hybridization (ISH) with biotinylated full genomic DNA probes of HPV types 1, 2, 3, and 4. The expression of PCNA (proliferating cell nuclear antigen) was examined using conventional immunohistochemistry. The aim was to test the hypothesis that HPV can reactivate PCNA, including in the host replication machinery. HPV DNA of the above types was detected in 60 of 90 verruca biopsies studied (66.7%): HPV 2 in 56 cases, HPV 1 in 2 cases, and HPV 3 in 2 cases. PCNA was expressed in all samples except two. The signal distribution of HPV DNA markedly differed from that of PCNA expression. ISH revealed strong HPV DNA signals in both the granular and the upper spinous cell layers, the most intense signals being detected in the upper epidermis. On the other hand, nuclear PCNA staining was present in the majority of parabasal and basal cells. Although strong PCNA signals within the wart lesions were found in the areas where HPV DNA was present, the PCNA positivity was almost invariably localized in the differentiated cells of the spinous cell layers, just below the HPV DNA-expressing cells. At the margins of the lesions, PCNA expression was still strong but disappeared abruptly towards the normal epidermis. HPV DNA-positive warts showed more intense expression of PCNA than did the HPV DNA-negative ones in this study. Our results indicate that PCNA induction is associated with the presence of HPV DNA, suggesting that HPV can reactivate PCNA, thus interfering with the host cell DNA replication machinery.