9295 smears, obtained from women attending three gynaecological hospitals for routine screening, were examined for human papillomavirus (HPV) types 6 and 11 and HPV 16 and 18 infections by filter in-situ hybridisation. The data were compared with cytological findings. In women with normal cytological smears HPV infection was identified in about 10% of women aged between 15 and 50 years and in less than 5% of those aged over 50. In women with abnormal smears (cervical intraepithelial neoplasia [CIN] I, II, and III and invasive cancer) HPV infection was detected in 35-40%; this rate seemed to be age-independent. the Peak incidence of CIN appeared several years after that of HPV infection. In women aged greater than 30 years it also declined earlier than did HPV positivity. The age-group distribution of women with CIN I, II, and III differed significantly from that of patients with invasive cancer. Only about a third of HPV-positive patients remained virus-positive, probably because of fluctuations in virus production and the insensitivity of the test system used. It is possible that filter in-situ hybridisation underestimates the total rate of HPV infections by a factor of 2 to 3.