Doubts have frequently been expressed about the effectiveness of the screening programme for cervical cancer in Britain. These doubts have been reinforced as a result of recent increases in mortality from this disease among younger women. In this paper we discuss trends in registration and mortality data, relate these to the level of screening, and conclude that screening may in fact have had a considerable impact on mortality rates. There is good evidence that in some age groups there has been a large increase in the incidence of carcinoma in situ of the cervix; it seems likely that the potential increase in cervical cancer incidence and mortality may have been partially prevented as a result of the screening programme. The extent of this effect cannot be quantified precisely because of uncertainties concerning the natural history of cervical cancer, differences in risk for different cohorts, and the possible effects of other factors. It is likely that incidence rates will continue to change, and it will be necessary to monitor these and the screening programme with some care in order to make the best use of the resources available for cervical cytology.