During the last 14 years, several articles have given contradictory reports on prevalence rates of squamous cell neoplasia of the cervix and vagina in diethylstilbestrol (DES)-exposed progeny. In 1984, the DES-Adenosis (DESAD) Project found that the incidence of cervical and vaginal intraepithelial neoplasia in DES-exposed daughters was twice as high as in a comparison group of unexposed women. This was also associated with a higher rate of herpes simplex virus and human papillomavirus infections. Possible hypotheses on the etiology of the increased incidence of squamous neoplasia are discussed.