Pre-operative mammograms and exact information on the pathological-anatomical extent of the tumour were available in 198 patients with carcinoma of the breast. The tumour as seen on the mammogram was measured and compared with the histological information. Five of the 198 carcinomas (2 1/2%) could not be seen on the mammogram. In 69.7% of cases, mammography was suspicious of a carcinoma and in 27.8% a lesion was found that required further investigation. In 67.7%, the mammographic and pathological tumour extent agreed within 3 mm. In 18.2% the tumour appeared more than 3 mm greater, and in 9.6% more than 3 mm smaller than real size; the error was greater than 30 mm in only 3.2%.