A case-control study of 497 women under age 40 diagnosed with cervical intraepithelial neoplasia (CIN) and 833 controls was done in the London area between 1984 and 1988 to examine whether known risk factors for invasive cervical cancer produced similar risks for CIN of different grades in young women. Cases of CIN III had a risk profile similar to that seen for invasive disease whereas CIN I cases were similar to the controls in all risk factors examined except a history of genital warts. Cases of CIN II were intermediate between the two. Among several indicators of sexual and reproductive behaviour, age at first childbirth and a history of multiple sexual partners were the strongest risk factors for CIN II and CIN III. Smoking had a strong and independent effect on the risk of CIN II and CIN III, but had only a limited effect for CIN I. Use of oral contraceptives was widespread in cases and controls, but length of use of oral contraceptives was not found to be a risk factor. A small protective effect of barrier contraception was observed.