Norway had until recently no organized screening programme for cervical cancer, but opportunistic screening was common. This study focuses on the effectiveness of treatment of pre-malignant cervical conditions (CIN III) on cervical-cancer incidence in the county of Sør-Trøndelag in Norway, prior to the introduction of organized mass screening. The study is based on cervical-cancer incidence rates during the years 1965-92 and treatment data for CIN III. The expected number of cervical-cancer cases prevented due to early intervention was expressed in a regression model with 2 unknown parameters: the probability, p, of cancer development in case of CIN III, and the time lag, t, between treatment and when clinical cancer would otherwise have been diagnosed. The estimated probability that a patient treated for CIN III would have developed cervical cancer if not treated was found to be approximately 20%, and the mean time delay was around 16 years. In the last period of study (1988-92), the incidence was reduced by nearly 40% of what would have been expected without early intervention. Based on equal treatment rates as in 1990, parameter estimates were used to predict future incidence reduction. Maximum effectiveness will be achieved around the year 2005, with a nearly 70% reduction. Opportunistic screening and treatment of CIN III seems to have had considerable influence on cervical-cancer incidence. The costs, however, are substantial over-treatment, since our results indicate that 4 of 5 women treated for CIN III would not progress into the invasive state.