PURPOSE To determine whether medium field strength (0.5 T) MR mammography is able to differentiate between benign and malignant lesions in the same way as a 1.5 T standard technique. MATERIAL In 40 consecutive female patients with nodular lesions, examinations were carried out at 1.5 T (2D-FFE, TR/TE/FA 200/3.9/80) and 0.5 T (3D-FFE, 24/3.4/40). RESULTS There was wide spread of the speed of enhancement of malignant tumours for both techniques (44%-145% with signal increase in the first minute of 42%-189%). In 15 out of 17 carcinomas, the rapidity of uptake and final degree of enhancement after contrast was higher during 0.5 T/3D measurements than it was for 1.5 T/2D images. Fibroadenomas and mastopathies showed similar enhancement characteristics for both techniques. Sharply defined "wash-out" was seen only in malignant lesions; it appeared ten times more frequently in the 0.5 T/3D examinations than at 1.5 T/2D. CONCLUSIONS The distinction between benign and malignant lesions can be made with more certainty using medium field strength and 3D-FFE sequence than using the 2D high-field standard technique.