Human papillomavirus (HPV) is an agent responsible for squamous cell tumors (verrucae, condylomata and papillomas) at various sites of the body, the oral cavity included. Due to the recently pointed-out association between these HPV lesions and the squamous cell dysplasias and malignancies in the uterine cervix, in the bronchus and in the larynx, the present work was carried out to assess, whether morphological signs (cytopathic effects of HPV) or HPV antigens could be found in lesions of oral squamous cell carcinomas. Biopsies from 40 cases of oral squamous cell carcinomas were surveyed by light microscopy with special emphasis on the presence of the histopathological features suggesting an HPV lesion, i.e. whether flat, inverted or papillomatous condylomas are present concomitantly with the malignancy. All specimens were also subjected to immunoperoxidase staining with anti-HPV serum to disclose the possible HPV antigens in the lesions. Morphological signs of the flat-type HPV lesion were found in 4 cases (10%), those of an inverted type in 3 cases (7.5%), and those of a papillomatous type in 9 cases (22.5%). Epithelial cells (mostly koilocytes) showing HPV-positive nuclei were disclosed in 5 papillomatous lesions, in 2 inverted lesions and in 1 flat lesion. The results suggest that HPV might be the agent involved in the development of at least certain special types of oral squamous cell carcinomas; albeit further confirmatory evidence with other techniques (DNA hybridization) is still needed.