A clinic for early detection of cancer of the uterine cervix has been in operation in Iceland since 1964, aimed until recently at the age-group 25-59. More than 85% of women in this age group have been screened at least once. Mortality from cancer of the cervix had been rising in Iceland, and continued to rise during the first few years of operation of the screening clinic. Since 1970, however, a more than two-fold reduction in mortality has been observed among women aged 25-59. There has been a similar decrease in incidence of tumours of stages II, III and IV. Both deaths and advanced tumours are largely confined to women who have never been screened. Alternative explanations are considered, but the only tenable explanation of the reduction in mortality is that it is a consequence of the introduction of a comprehensive screening programme.