Human papillomavirus (HPV) infection of cervical intraepithelial neoplasia (CIN) and invasive cervical carcinoma was investigated using in situ DNA-DNA hybridisation on histological sections of formalin fixed, paraffin embedded tissue to assess the technique's sensitivity and to assess retrospectively the association between HPV16 and invasive cervical carcinoma. HPV DNA was detected in 16 of 33 biopsy specimens of CIN. Cells containing viral DNA were more numerous than those positive for viral structural proteins. HPV DNA was also present in less differentiated cells deeper in the epithelium. The detection rate in CIN was lower than that reported for other hybridisation techniques such as Southern blotting. In a retrospective study of biopsy specimens of invasive squamous carcinoma of the cervix HPV16 DNA, the virus most commonly associated with cervical malignant disease, was found in 20 of 25 cases, including those dating from as far back as 1932. The level of sensitivity was similar to that reported for other hybridisation techniques. DNA positive cells were focally distributed in the invasive tumours, and most tumour cells were negative for viral DNA, a result consistent with the low copy number found in malignant cells. It is concluded that HPV16 is not a new virus but that its prevalence is a result of changes in sexual behaviour and that in situ hybridisation is useful in the localisation of HPV DNA replication in CIN and invasive carcinoma.