Iatrogenic displacement and mechanical transport of epithelial cells to the sentinel node (SN) has been suggested to result in false-positive findings in breast cancer patients, but little biologic evidence has yet been presented for this hypothesis. As malignant nuclei are larger than benign ones, nuclear morphometry of SN isolated tumor cells (ITC) could provide relevant information with regard to the malignant origin-or-not of epithelial cells in the SN. In patients with primary invasive breast cancer and SN ITC with (N=16) or without (N=45) non-SN involvement, nuclear morphometry was performed on the primary tumor as well as on the ITC in the SN. Nuclear size in the primary tumor was compared with that in the corresponding ITC. Patients with SN micrometastases (N=30) and SN macrometastases (N=30) served as controls. Nuclear size of ITC was significantly smaller compared with nuclear size of the corresponding primary tumor (P<0.0001). In contrast, there were no differences in nuclear size between SN micrometastases and macrometastases on the one hand and their corresponding primary tumors on the other. In addition, a subgroup of cases (10/61, 16%) with benign morphometric features of SN ITC nuclei (small and isomorph) could be discerned that had no non-SN metastases. In conclusion, nuclei of SN ITC are significantly smaller compared with the corresponding primary tumor and are often not associated with non-SN involvement. This supports the hypothesis that some of these deposits could represent benign epithelium or degenerated malignant cells lacking outgrowth potential.