In a prospective study 42 women, diagnosed as having low grade cervical intraepithelial neoplasia (CIN), made a total of 281 clinic visits over a 45 month period. At each visit, they were subjected to cytological and colposcopical examination and samples were taken for human papillomavirus (HPV) DNA hybridization studies and for the detection of non-HPV infections. HPV types 16 and/or 18 were found in 25% of all the samples tested and these virus types were detected in five of six (83%) women whose lesions progressed compared to seven of 14 (50%) of those whose lesions regressed. The presence of HPV DNA was not a good prognostic indicator of progression since half of those whose disease regressed also harboured these viruses at some time. The recording of non-HPV infections almost 10 times more often in the women whose disease regressed than in those whose disease progressed could probably be accounted for by the former having a larger number of follow-up visits. Nevertheless, the significance of non-HPV infections also remains unclear.