Cytogenetically unrelated clones are found in half of all carcinomas of the breast and also in the epithelial fraction of many benign breast tumors. The chromosomal aberrations thus detected are clearly nonrandom and appear to be the same as those often seen in other tumors as sole karyotypic anomalies. Clonal chromosome abnormalities are not found in histologically normal breast tissue. Cytogenetically unrelated clones may be found in both primary tumors and secondary lesions, be it within the same breast (multifocal carcinomas), in the contralateral breast (bilateral carcinomas), or in lymph node or other metastases. The aberrations are present in topologically separate tumor domains and may confer on the cells that harbor them different types of cancer-specific behavior, such as the ability to metastasize and invade locally. Whereas the available evidence thus strongly indicates that the cells carrying clonal karyotypic aberrations all are part of the neoplastic parenchyma, it is less certain whether cytogenetic polyclonality actually signifies a multicellular tumor origin, although we think that this is the explanation that best accommodates the cytogenetic data. But even if it should eventually be shown that the seemingly unrelated clones have some submicroscopic tumorigenic mutation in common, the observed karyotypic heterogeneity is remarkable and goes far beyond what one has become accustomed to from most other tumor types. To understand how the various clones interact during mammary carcinogenesis will be a major task in future breast cancer research.