In the six years from 1979 to 1985, a total of 735 women were referred to the Department of Radiation Oncology, Westmead Hospital, for the management of primary breast cancer. Of these, 218 had undergone mammography after the discovery of their breast tumour. In 95 of those patients with breast cancer the mammogram had failed to define the carcinoma. For almost 50% of the patients with a negative result of a mammogram, the negative report result had, in our opinion, led to a delay in definitive management and to the re-presentation of those patients at a later date with a disease of poorer prognosis. A false-negative result of a mammogram was particularly likely to be obtained in young premenopausal women with small tumours. When these women finally presented with breast cancer, the tumour was larger, and the prevalences of pathological involvement of the axillary nodes and of locally advanced disease were significantly greater. The misuse of mammography as a diagnostic tool in patients with symptomatic disease is dangerous in that it has a significant false-negative rate which carries with it the serious risk of postponing a biopsy.