Whether macroscopically distinct carcinomas in the same breast are clonally related (multifocal breast carcinoma) or unrelated (multicentric breast carcinoma) is no longer only a scientific-pathological issue but, because different therapeutic strategies may be preferable for cases with intramammary metastatic disease compared with cases of multiple primary breast carcinomas, one that may have profound clinical implications. We studied the evolutionary relationship among macroscopically distinct, ipsilateral breast carcinomas by cytogenetic analysis of 26 tumorous lesions from 12 patients. Sixteen of the 26 foci (62%) were found to contain clonal chromosome abnormalities. Two carcinoma foci were karyotypically abnormal in each of seven patients. Four of these cases had an evolutionarily related, cytogenetically abnormal clone in the two lesions from the same breast, whereas the remaining three cases had completely different clonal karyotypic aberrations in the separate foci. These results, together with our previous findings in five other informative cases, show that multiple, synchronous breast tumors sometimes arise through intramammary spreading of a single primary carcinoma, whereas on other occasions they are the result of the simultaneous emergence of pathogenetically independent carcinomas within the breast. In the total material, an association was seen between the proximity of the foci and the likelihood of them being karyotypically related.