A prospective study of 100 women with cytological and colposcopic evidence of mild cervical atypia consistent with cervical intraepithelial neoplasia (CIN) grade I was started in October, 1983. 26% of early preinvasive cervical lesions progressed to histologically proven CIN III. Spontaneous regression of mild cervical atypia occurred in only 11 cases, and in 4 of these CIN recurred. The overall prevalence of human papillomavirus type 16 (HPV 16) in the study group, detected by filter DNA-DNA hybridisation of a cervical cytological specimen, was 39%. However, 22 of the 26 (85%) cases of progressive disease were positive for HPV 16. Detection of HPV 16 may be a non-invasive way of identifying women at high risk of rapid progression of mild cervical atypia to CIN III.