Evidence has accumulated in recent years that relates certain types of human papillomaviruses (HPV) to the development of some tumors. We use in situ hybridization to study DNA from HPV 6/11, 16/18 and 31/33/35 in 6 squamous papillomas, 18 hyperkeratotic/acanthosis lesions with and without dysplasia (5 and 13 cases respectively) and 27 squamous cell carcinomas of the oral mucosa. HPV DNA was found in 66% of squamous papillomas, 38.4% of hyperkeratotic/acanthosis lesions without dysplasia, 60% of epithelial dysplasia and 37% of squamous cell carcinomas. HPV DNA 6/11 was the most common type found, and in squamous cell carcinomas HPV DNA appear more frequently in well differentiated tumors.