The relation between cigarette smoking and cervical intraepithelial neoplasia, grade III (CIN III), and cervical cancer was examined among a cohort of 6,812 women in Tromsö, Norway, between 1980 and 1989. During the 52,844 person-years of observation, 185 incident cases (177 women with CIN III and eight with cervical cancer) were recorded in the regional pathology registry. The age-adjusted incidence rates of CIN III and cervical cancer were 267/100,000 person-years among women who had never smoked, 183/100,000 person-years among exsmokers, and 476/100,000 person-years among current smokers. A multivariate model containing terms for age, marital status, and frequency of intoxication yielded a relative rate for current smokers compared with nonsmokers of 1.5 (95% confidence interval 1.0-2.2). Statistical trend tests for the number of cigarettes smoked per day (never, 1-14, and greater than or equal to 15 cigarettes), years of smoking (never, 1-9, and greater than or equal to 10 years), and age started smoking (less than 16, 16-18, 19-21, and greater than or equal to 22 years) all yielded significant results. These findings support the opinion that CIN III and cervical cancer are a smoking-related disease.