In order to determine if there are morphologically identifiable characteristics between malignant cells obtained from a primary cancer and its metastasis the nuclear diameter was used as an indicator of the degree of malignancy, since there is good correlation between nuclear size, DNA content, and chromosome numbers. The nuclear diameter of primary and metastatic mammary carcinoma cells, obtained by cytologic aspirates, was measured by ocular micrometry. The purpose was to investigate whether a cell population at the primary site developed, at the metastatic sites, a population with the same nuclear size or one having larger and more anaplastic nuclei. One hundred eighty-five patients with infiltrating ductal carcinoma of the common variety were examined. The primary cancer and axillary nodal metastasis were examined in 97 patients before treatment. Thirty had cytologic examination of the breast cancer, as well as of the metastasis, which developed 1 to 14 years after treatment. Eleven were examined before radical breast irradiation and again at the time of relapse in the breast. Forty-seven had bilateral synchronous mammary carcinoma and both primary cancers were studied. The data presented indicate that there is extreme similarity between the nuclear diameters of the primary tumor and its metastasis. This similarity persists for several years regardless of both the location of the recurrence or radical irradiation. These results support the view that the majority of tumors are monoclonal in origin. The clone that invades the metastatic site appears to be the same as the one that initiated the primary cancer. In contrast, the nuclear diameters of cell populations obtained from synchronous bilateral breast cancer were dissimilar, indicating that they arose from separate clones of malignant cells.