Cyclin-D1 (CD1) expression was analyzed in human mammary carcinomas by immunohistochemical (IHC) and flow-cytometry (FCM) methods: 52.5% and 50% of cases were strong expressors of CD1 by IHC and FCM analysis respectively. The percentage of CD1-positive cells was especially high in node-negative (N-) estrogen-receptor-positive (ER+) tumors, probably as a consequence of CD1 induction by estrogens in steroid-responsive tissues. However, CD1 expression was not related to ER positivity in node-positive tumors (N+). An interesting relationship between CD1 expression and H3-thymidine labelling index (H3Td-LI) was also found: CD1 and H3Td-LI were unrelated in N- tumors, while high CD1 expression was observed in N+ tumors with high DNA synthesis, as assessed by H3Td-LI. The combined measurement of DNA and CD1 showed that 27 specimens were aneuploid, 19 of them (19/27; 70%) strongly expressing CD1. Further studies are needed to clarify the role of CD1 in DNA abnormality of breast tumors. However, we cannot exclude that the CD1 may be differently de-regulated in the last phase of tumor progression, and that CD1 over-expression may contribute to the aneuploidy of mammary carcinomas.