A review of the mammograms of 871 patients with breast cancer from a symptomatic clinic performed at the General Hospital, Birmingham between 1980 and 1988 revealed an overall false negative rate of 8.6%. There has been a steady fall in the number of cancers missed per year from the commencement of review, and this reduction has been more consistent with improvements in radiographic equipment and technique, particularly the introduction of a radiographic grid. In half of the cases the tumour was missed because no radiological abnormality was detectable, even on reviewing the films, and this rate has remained remarkably stable over the study period, emphasizing the importance of a clinical examination in symptomatic women. Comparison of the histological diagnoses revealed similar percentages of ductal carcinomas, 89.6% in the true positive group compared with 85% in the false negative group. Of the false negative carcinomas, 5.5% were medullary tumours compared to 0.8% in the true positive group.