Human papillomavirus space (HPV) an Epstein-Barr virus (EBV) are epitheliotrophic viruses invading the upper aerodigestive tract. HPV produces a diversity of benign or malignant tumors and EBV is known to be a major etiologic factor of nasopharygeal carcinoma. We evaluated the HPV and EBV infection and co-infection rate in nasopharyngeal carcinoma and squamous cell carcinoma of larynx and the significance of HPV infection of different subtypes in the tumorigenesis of head and neck carcinoma. Thirty-four cases of nasopharyngeal carcinoma and thirty-nine cases of laryngeal squamous cell carcinomas were analyzed for the detection of HPV DNA and EBV RNA (EBER-1) by in situ hybridization technique. The following results were obtained : 1) HPV DNA was detected from 5 cases of nasopharyngeal carcinomas (21.73%) and 15 cases of laryngeal carcinomas (38.46%), HPV type 18 was most commonly detected from as many as 12 cases. 2) EBER-1 was detected from 29 cases of nasopharyngeal carcinomas (85.29%) and 7 cases of laryngeal carcinomas (17.95%). 3) The co-infection rate was 11.76% (4 cases) in nasopharyngeal carcinomas and 10.26% (4 cases) in laryngeal carcinomas. In conclusion, EBV infection is strongly suspected to be the cause of nasopharyngeal carcinomas, while HPVs, especially the HPV subtype 18, are thought to be in a part, the causes for nasopharyngeal carcinoma and squamous cell carcinoma of larynx. For the co-infected cases, however, whether these viruses have ""affinity"" to head and neck tumor tissues or the two viruses together have been the causal factor is uncertain.