The optimal frequency of breast cancer screening has been a subject of debate since the inception of the UK National Breast Screening Programme (NHSBSP). This paper reports on the only randomised trial directly comparing different screening intervals. 99389 women aged 50-62 years who had been invited to a prevalent screen were randomly allocated after the scheduled prevalent screen date to the study arm (invited to three further annual screens), or to the control arm (invited to the standard single screen 3 years later). 37530 women in the study arm and 38492 in the control arm had attended the prevalent screen. The endpoint was predicted breast cancer deaths. The prediction was based on both the Nottingham Prognostic Index (NPI) and a similar method derived from survival data from a series of tumours in the Swedish Two-County screening trial (2CS). Both indices were based on the size, lymph node status and histological grade of the invasive tumours diagnosed in the two arms of the trial. The pathology of the cases diagnosed was subject to review by two pathologists using standard criteria. The tumours diagnosed in the study arm were significantly smaller than those diagnosed in the control arm (P=0.05). The relative risk of death from breast cancer for the annual compared with the 3-yearly group was 0.95 (95% Confidence Interval (CI): 0.83-1.07, P=0.4) using the NPI and 0.89 (95% CI: 0.77-1.03, P=0.09) using the 2CS. Shortening of the screening interval in this age group is predicted to have a relatively small effect on breast cancer mortality. Improvements to the screening programme would be targeted more productively on areas other than the screening interval, such as improving the screening quality.