We conducted a case-control study of cigarette smoking and dysplasia and carcinoma in situ of the uterine cervix. Cases were black women 17 to 55 years of age who were attending a dysplasia clinic and had biopsy-confirmed cervical pathologic conditions. Controls were women who were attending the family planning clinic at the same hospital and who had at least two normal Papanicolaou smears. Results were adjusted for age, number of sexual partners, age at first intercourse, socioeconomic status, and oral contraceptive use. Cigarette smoking was significantly associated with carcinoma in situ, severe dysplasia, and mild-moderate dysplasia (relative risks, 3.6, 3.3, and 2.4, respectively). Cumulative exposure to cigarette smoking (as measured by pack-years smoked) was strongly related to the risk of these conditions; women with 12 or more pack-years of exposure had relative risks of 12.7, 10.2, and 4.3, respectively, for the three conditions. There was some evidence that the risk was greatest in women who began smoking in their early teenage years. A reduction in the risk of cervical cancer appears to be another inducement for young women not to smoke.