BACKGROUND: Recent reports suggest that the reduction in mortality achieved by the UK national cervical screening programme is too small to justify its financial and psychosocial costs, except perhaps in a few high-risk women. METHODS: We analysed trends in mortality before 1988, when the British national screening programme was launched, to estimate what future trends in cervical cancer mortality would have been without any screening. FINDINGS: Cervical cancer mortality in England and Wales in women younger than 35 years rose three-fold from 1967 to 1987. By 1988, incidence in this age-range was among the highest in the world despite substantial opportunistic screening. Since national screening was started in 1988, this rising trend has been reversed. INTERPRETATION: Cervical screening has prevented an epidemic that would have killed about one in 65 of all British women born since 1950 and culminated in about 6000 deaths per year in this country. However, these estimates are subject to substantial uncertainty, particularly in relation to the effects of oral contraceptives and changes in sexual behaviour. 80% or more of these deaths (up to 5000 deaths per year) are likely to be prevented by screening, which means that about 100000 (one in 80) of the 8 million British women born between 1951 and 1970 will be saved from premature death by the cervical screening programme at a cost per life saved of about pound 36000. The birth cohort trends also provide strong evidence that the death rate throughout life is substantially lower in women who were first screened when they were younger.