Cytogenetic aberrations have until now not been reported in mammary hamartomas. The finding of multiple, karyotypically abnormal clones in short-term cultures from 2 such tumors supports the interpretation that these are genuinely neoplastic lesions. The deletion del (3) (p13p14) and trisomy 18, both known to occur as primary chromosome abnormalities in breast carcinoma, were among the clonal changes in one case each. Since both tumors had structural abnormalities of 3p13-14, admittedly leading to different derivative chromosomes, rearrangement of this region might represent a unifying feature in the genesis of mammary hamartomas.