The Icelandic Cancer Society launched a screening programme for cervical cancer in June 1964. The aim was to lower the incidence and mortality rates by screening the age group 25-69 at 2to 3-year intervals. This report analyses the trends in invasive and pre-invasive disease and the distribution of stage and histology, and also evaluates the attendance, the target age group and the screening interval. Before screening, the incidence and mortality rates were on the increase but both have since fallen significantly. Screening greatly affected the rate of microinvasive and stage rll squamous-cell carcinomas but not the rate of adeno-and adenosquamous carcinoma. The mean age at detection of invasive cancer has decreased significantly and cancer has become practically non-existent among correctly screened subjects over the age of 69. Among younger women the rates of moderate and severe pre-invasive lesions at first visit increased significantly after 1980. The rate of these lesions was fairly consistently high only I year after the first normal visit. It is concluded that organized screening, coordinated with spontaneous activity, is effective in reducing the risk of cervical cancer. Regular high attendance and strict follow-up of abnormal cases is a prerequisite for good screening results. Screening should preferably start at about the age of 20 and extend to 60-69 years of age, depending on the number of negative smears by that age. Screening can safely start with a screening interval of 2 to 3 years, but this interval can probably be extended to 4 or 5 years at older ages.