Between 1979 and 1981, 45130 women in Edinburgh aged 45-64 were entered into a randomised trial of breast cancer screening by mammography and clinical examination. The initial attendance rate was 61% but this varied according to age and socioeconomic status and decreased over succeeding years. The cancer detection rate was 6·2 per 1000 women attending at the first visit; the rate fell to around 3 per 1000 in the years when mammography was routinely repeated and to around 1 per 1000 at the intervening visits with clinical examination alone as the screening method. After 7 years of follow-up the mortality reduction achieved was 17% (relative risk=0·83, 95% Cl 0·58-1·18), which was not statistically significant, even when corrected for socioeconomic status. In women aged 50 years and over a mortality reduction of 20% was achieved.