Paired blood and breast tissue samples from 96 patients undergoing surgical excision of a breast lesion were subjected to DNA fingerprint analysis using the minisatellite probes 33.6 and 33.15. The 'fingerprints' of the blood and breast DNA were compared. DNA fingerprint changes seen were classified as band additions, band deletions or changes in band intensity. Significantly more DNA fingerprint changes were seen in malignant than in benign lesions (probe 33.6, P<0.01; probe 33.15, P=0.01; both probes, P = 0.04). The frequency of the changes detected in the breast cancers was unrelated to tumour grade or the presence of invasion. DNA ploidy was measured in 70 of the breast carcinomas. DNA fingerprint changes were seen in 41 of these, but flow cytometry detected aneuploidy in only 29 tumours (P=0.04). The DNA fingerprinting technique is unlikely to be useful in differentiating between benign and malignant breast lesions as changes were detected in one of the fibroadenomas studied. However, it may be a useful technique in screening tumour DNA for abnormalities suitable for more detailed analysis.