Macroscopic measurements and histometrical estimates of cell cytoplasmic (Cx) and nuclear areas (Nx) were made on 24 primary and 37 metastatic tumors from patients with human breast carcinoma. Within the small metastases, the cancer cells were smaller and showed greater variation in size (CVC, CVN) than those in their respective primary tumors. In the larger metastases, the cells were larger and more uniform in size. Studies of primary tumors showed that both Cx and Nx of tumor cells bore a direct relationship to the gross size of the primary tumor. Comparisons of measurements from the 14 primary tumors with metastases and the 10 primary tumors with no metastases revealed no significant differences between the groups. In all tumors, Cx bore a predictable and direct relationship to Nx.