It still needs to be verified whether multiple syncronous homolateral and bilateral breast cancers represent intramammary spread of a single tumor or two or more separate neoplastic events. To clarify this problem, we studied the biopathological profile of 46 homolateral and 20 bilateral cases. The cancers were always surgically removed and processed at the same time. The expression of estrogen receptors (ER), progesterone receptors (PR), MIB 1, p53, and c-erbB-2 was determined. Computer-assisted image analysis (CAS 200) was used to evaluate ER, PR, MIB 1, and p53. The histological concordance was 95.6% in homolateral and 50% in bilateral cases. The immunophenotype profile of multiple homolateral neoplasms showed a concordance between 93.47% for ER and 78.26% for p53. The results were statistically significant for all parameters except for p53. In bilateral cancers, there was a significant statistical concordance for ER. These data strongly suggest that both mechanisms may exert an influence and, in particular, that in the majority of homolateral carcinomas, there may be intramammary spread of tumor cells. In multiple bilateral tumors, however, the great diversity of the histological aspects and the differences in the immunophenotype pattern suggest that the vast majority of these may constitute independent multiple events.