The aim of this paper was to assess whether the morphological appearance (i.e. histological tumor type and histological grade) of simultaneous invasive breast carcinoma foci is heterogeneous, since it is known that adjuvant therapy is established according to these parameters. Patients with simultaneous breast tumors in which only the features of the largest neoplastic focus are reported could thus be undertreated. A retrospective study of 418 cases of breast carcinomas was conducted over a 3-year period. The histological tumor types and histological grades of multifocal/multicentric carcinomas in each tumor focus were compared, and mismatches among foci were recorded. Ninety-one of the 418 cases reviewed had multiple carcinomas (21.77%). A comparison between multiple synchronous tumor foci revealed that their histological type was different in 12.08% of the cases. Mismatches among foci were also observed in 9.89% of the cases when evaluating the histological grade, and 5 out of 9 additional tumor foci with a different grade from the largest (index) tumor (55.55%) displayed a higher grade compared to the index tumor. Since the histological tumor type and histological grade of the individual foci may vary considerably within the same tumor and the additional foci may be of higher grade than the index tumor, we believe that reporting morphologic parameters with more unfavorable characteristics in addition to the parameters of the index tumor is imperative.