In 1959, the Norwegian Cancer Society started a research project in Ostfold county to assess the effect of a mass screening program for cervical cancer. The program comprised 5 screening rounds during the period 1959-77. At each screening a clinical gynecological examination was performed in addition to the cytological test. A total of 45,960 women aged 25-59 years at the first screening were invited to participate. The follow-up, which was based on the data base of the Cancer Registry of Norway, lasted until December 31, 1982. For the follow-up period as a whole, the observed incidence and mortality were reduced to 78% and 83% of the expected values, respectively. Through the initial phase of the program there was a cumulation of patients with early-stage disease, while the incidence of advanced disease and mortality were reduced. In the later part of the follow-up period, a decrease in incidence of early and advanced disease as well as in mortality was observed. Women not participating in the screening program had a significantly higher risk of cervical carcinoma than the reference population, and a less favorable stage distribution as well. Among participants in the screening program, the reduction of risk was dependent upon the number of previous negative smears. Women with 5 previous negative smears had a risk of disease which was only 18% of that expected. Problems concerning choice of reference population, migration, attendance rates, sensitivity and specificity of the cytological tests and intervals between screenings are discussed. Also, the probable conversion rates of pre-cancerous lesions to cancer and the health hazards involved in possible over-treatment of pre-cancerous cases which otherwise would have regressed, are considered. A dynamic model is suggested. In order to achieve a reduction of mortality in the short term, the efforts of the program should initially be concentrated on older women. To achieve a reduction of incidence in the long run, younger women should gradually be included. Finally, special efforts should be made to increase attendance rates among high-risk women.