Nuclear pleomorphism is a fundamental feature in evaluating the aggressiveness of ductal carcinoma in situ (DCIS) of the breast. In this study, pure DCIS and the in situ component (IS-comp) of invasive duct carcinoma (IDC) are compared in order to verify if these are two different entities or the same process observed at different times during its evolution. Five cases of pure DCIS and nine of IDC with extensive in situ component were selected. They were moderately and poorly differentiated. 30 nuclei for each DCIS, and 30 nuclei for both the in situ and invasive component of each IDC were studied; thus, a total of 720 nuclei were submitted to the SAM (Shape Analytical Morphometry) analysis, which enables a numerical expression not only of dimensions (area, perimeter, diameter) but also of nuclear contour irregularities and nuclear shape distortions. Univariate statistical comparisons were carried out between the nuclei of: (1) DCIS and in situ component of invasive duct carcinoma, (2) DCIS and the invasive component of infiltrating carcinoma and (3) between the in situ and invasive component of infiltrating carcinoma. Multivariate analysis was utilized to compare nuclei of DCIS with the in situ component of IDC. The in situ features of each tumor were also evaluated with the mitotic index (MI). Nuclei of pure DCIS resulted significantly larger (p < 0.001) and with a more regular shape (p < 0.001) than those of the in situ component of IDC. No differences were observed between the nuclei of the in situ and the invasive component of infiltrating carcinomas. Multivariate statistical analysis discriminated 77% of nuclei of in situ proliferation when both G2 and G3 tumors were considered, and 80% when only G3 tumors were considered. In conclusions morphological differences exist between pure DCIS and the in situ component of IDC, which may be an expression of their biological behavior; moreover, these morphological differences seem to have a better discriminating power within the same histological grade.