Dynamic MR mammography (MRM) has high sensitivity for the demonstration of carcinomas. 63 carcinomas were examined by this technique and all showed a signal increase of more than 90% 60 seconds after the injection of 0.1 mmol/kg Gd-DTPA and an almost constant signal intensity (carcinoma plateau) for a further 7 minutes. These characteristic changes applied only to active tumour areas and not to necrotic or fibrotic tumour regions. A possible explanation is the altered vascular architecture due to the growth of malignant tumours. In the presence of a suspect mammogram a carcinoma can be excluded with a high degree of certainty in the absence of contrast enhancement. Signal enhancement in 54 histologically proven cases of non-proliferative mastopathy and in 52 cases of proliferative mastopathy could be reliably distinguished from carcinomas (p less than 0.001). In rare cases of proliferative mastopathy the signal changes were suspicious of a carcinoma. Normal signal increase does not exclude proliferative mastopathy with certainty. Patients with increased contrast uptake should be regarded as "high-risk" and examined at regular intervals.