Four groups of females 19 years of age were studied in order to investigate the sexual factors in the development of cervical intraepithelial neoplasia (CIN) in teenagers. Group A consisted of 101 females treated for CIN, group B of 55 females with inflammatory atypic cytology (Pap cl. II), group C of 73 women with cervical chlamydial infection, and group D of 114 females with normal cervical cytology and negative chlamydial isolation. Subjects in group A had had significantly more deliveries and legal abortions than the subjects in the other groups. They used also more often oral contraception and had been treated more often for vaginal infections than the subjects in the other groups. Coital activities in group A had started at a significantly younger age than in group D; in the latter group 40% had no history of sexual intercourse. Also the numbers of partners and the coital frequency in group A were significantly higher than in group D. Our study provides further evidence for the theory that CIN is, at least partly, a sexually transmitted disease.