A series of 22 squamous cell carcinomas (4 cases grade 1; 11 cases grade 2; 7 cases grade 3) of the oral cavity (13 cases), (naso-)pharynx (5 cases) and larynx (4 cases) were studied by conventional light microscopy and filter (dot blot) hybridization for the detection of human papillomavirus (HPV) DNA. In 4 carcinomas, tumour-free tissue from the resection margins of the surgical sample was examined in addition to the tumour specimen. The same kind of investigation was carried out on 4 oral leukoplakias without dysplasia. All cases were thoroughly examined for HPV-associated cytopathic effects (koilocytosis). In all cases, material was obtained for DNA extraction followed by dot blot hybridization. DNA hybridization was carried out under stringent conditions with mixed probes of HPV 6/11 as well as HPV 16/18. Koilocytosis was observed in 10/22 carcinomas (45%, 4/4 G1 tumours, 6/11 G2 tumours, none out of 7 G3 tumours) and 3/4 leukoplakias. Koilocytosis always occurred at the tumour surface or the surface epithelium immediately adjacent to the tumour. HPV DNA was found in 8/22 carcinomas (36%, 2/4 G1 tumours, 5/11 G2 tumours, 1/7 G3 tumours). We observed HPV 16/18 infections in 3 cases and HPV 6/11 infection in 1 case. The other 4 cases were positive under relaxed conditions and, thus, could not be grouped into one of the examined types of HPV infections. In 4 carcinoma cases, tumour tissues and resection margins were examined. 3/4 cases showed concordant findings, i.e. in 2 cases tumour tissue and tumour-free mucosa (1–2 cm distant to the tumour) were positive for HPV, 1 case was negative in both samples. In 6/8 cases positive for HPV, HPV DNA detection corresponded to the observation of intensive koilocytosis. The leukoplakias were seen to harbour HPV DNA in 3 cases (1 case: HPV 6/11; 1 case: HPV 16/18; 1 case: positive under relaxed conditions). We did not observe HPV DNA in normal mucosal tissues. Our findings provide further evidence for the presence of HPV infections in tumours of the upper respiratory and digestive tract. Prospective studies now have to clarify the biological importance of HPV infections in this group of human cancer.