A series of routinely processed, paraffin-embedded biopsies from 73 surgically treated oral precancerous lesions (OPL) (22 cases), and oral squamous cell carcinomas (SCC) (51 cases), was first screened using an in situ DNA hybridization technique with a human papillomavirus (HPV) DNA probe cocktail containing the 35S-labelled DNA of HPV types 6, 11, 13, 16, 18 and 30. The specific HPV types in lesions shown to contain HPV DNA in this procedure were further analysed by using in situ hybridization and the 6 HPV DNA probes separately. A total of 12/73 (16.4%) of the lesions proved to contain HPV DNA; 6/51 (11.8%) carcinomas and 6/21 (28.6%) dysplasias. The most frequent sites of HPV DNA-positive lesions were palate (4/7; 57%), followed by the floor of the mouth (2/8; 25%), the tongue and gingiva (11.8%). HPV 13 or HPV 30 were not found in any of the lesions studied. HPV 11 DNA was demonstrated in 2 mild dysplasia lesions, but not in carcinomas. One additional mild dysplasia proved to contain HPV 6 DNA. HPV 16 DNA was present in 5 biopsies; 3 carcinomas and 2 dysplasias. In one of the HPV 16-positive carcinomas, HPV 18 DNA was simultaneously present. HPV 18 alone was found in 3 additional carcinomas and in one moderate dysplasia lesion. The results confirm the recently reported evidence on HPV involvement in OPL and oral cancer. The implications of these findings are discussed in terms of the possible HPV etiology of oral SCC.(ABSTRACT TRUNCATED AT 250 WORDS)