Interval cancer rates are a major determinant of the success of a screening programme. In the Swedish two county study, on which the United Kingdom programme is based, a 39% reduction in mortality was observed in screened women aged 50-64. Using data from the Swedish study, the relationship between interval cancer incidence and the likely future effect on breast cancer mortality was quantified. In East Anglia, as elsewhere in the United Kingdom, interval cancers rates are nearly double those obtained in Sweden: interval cancer rates in the first, second, and third years respectively, after a negative screen were 24%, 59%, and 79% of the expected underlying incidence in the absence of screening. The corresponding figures from the two county study were 17%, 30%, and 56%. From these it was estimated that the mortality reduction in East Anglia will be 21%, which is lower than the 35% observed in invited women in this age group in the Swedish two county study and the 25% specified in the Health of the Nation target. In a rereading exercise, using screening mammograms from women who were screen normal, who had screen detected cancers, or who subsequently developed interval cancers, four out of five radiologists recommended recall for around 70% of the original mammograms (classed as screen normal at time of screening) from 33 interval cancers. This suggests that sensitivity is a contributory factor to the higher interval cancer rates in East Anglia.