Blood flow was measured by colour Doppler ultrasonography in 33 fibroadenomas of size 5-31 mm and 28 malignant breast masses of 8-37 mm visible on ultrasonography. There was detectable blood flow in 11 fibroadenomas and 21 cancers (P < 0.01). The median peak systolic frequency was 1.0 (range 0.25-2.0) kHz in malignant tumours and 0.5 (range 0.2-1.25) kHz in fibroadenomas. Blood flow was not detected in the eight fibroadenomas of less than 13 mm but was present in five of seven cancers smaller than this. Five of six carcinomas with three or more detectable vessels were classified as grade III on histological examination, compared with two of 11 carcinomas with one or two vessels. Similarly, five of seven cancers with three or more detectable vessels had axillary lymph node metastases compared with two of 13 with only one or two vessels. Detectable blood flow in breast masses is more common in cancer than in fibroadenoma and is highly suggestive of malignancy if the mass is less than 13 mm in size. Malignant tumours with a larger number of vessels are more likely to be of high grade and to have associated axillary node metastases.