The literature dealing with the natural history of cervical intraepithelial neoplasia (CIN) since 1950 is reviewed, in particular from the viewpoint of regression, persistence, and progression. When stratified into the various grades of severity, the composite data indicate the approximate likelihood of regression of CIN 1 is 60%, persistence 30%, progression to CIN 3 10%, and progression to invasion 1%. The corresponding approximations for CIN 2 are 40%, 40%, 20%, and 5%, respectively. The likelihood of CIN 3 regressing is 33% and progressing to invasion greater than 12%. It is obvious from the above figures that the probability of an atypical epithelium becoming invasive increases with the severity of the atypia, but does not occur in every case. Even the higher degrees of atypia may regress in a significant proportion of cases. As morphology by itself does not predict which lesion will progress or regress, future efforts should seek factors other than morphological to determine the prognosis in individual patients.