Twenty-one cases of verrucae vulgaris (oral warts) were investigated for human papillomavirus (HPV)-group specific antigen by immunocytochemistry and for HPV types 1, 2, 4, 6, 11, 16, 18 and 57 by DNA in situ hybridization with biotinylated probes. Twelve (57%) cases demonstrated the presence of HPV-group specific antigen. Fifteen (71%) cases showed the presence of HPV DNA, 13 of which (87%) demonstrated both HPV types 2 and 57 in the same cells and 2 of which (13%) demonstrated only HPV 2. Six cases were negative for HPV 2 and 57 and all 21 cases (100%) were negative for HPV types 1, 4, 6, 11, 16 and 18. Results indicate the association of a new and as yet unidentified HPV type, closely related to HPV 2 and 57, with oral warts. The identification of both cutaneous type HPV 2 and another type closely related to HPV 2 and 57 in oral verrucae on keratinized and non-keratinized mucosal surfaces indicates the possibility of a latent infection; three patients had a history of warts on their hands, suggesting autoinoculation. This study indicated that future investigations of oral warts, based on a correlation of clinical and histological features with HPV types by DNA in situ hybridization, are called for.