PURPOSE: Despite the fact that the primary factor to determine the prognosis of breast cancer is the metastatic lesion rather than the primary tumor, most studies concerning the prognostic factors related with tumoric biological behavior have focused on the primary tumors. A better understanding of changes of biological behavior in the metastatic lesions will provide a clue to more effective and rational approaches for treating metastatic breast cancer. METHODS: This study was designed to investigate the biological characteristics of metastatic cancer cells in breast cancer and to compare them to those of the primary tumors. Eighty-two breast cancer patients with metastatic axillary lymph nodes were selected for study. The evaluated tumoric biological characteristics used in this study were histologic grade, estrogen receptor, progesterone receptor, bcl-2, c-erbB2, p53, and P-glycoprotein. Evaluations were carried out with H-E and immunohistochemical stainings. The subjects were divided into positive cases and negative cases, according to extent and degree of staining. McNemar's test and Spearman's rho correlation coefficient were used for statistical analysis and cases showing a p-value of 0.05 or less were taken as being statistically significant. RESULTS: The results were as follows: 1) Metastatic nodes showed higher histologic grade than primary tumors. 2) No significant pattern was observed concerning changes in biological characteristics, including estrogen receptor, progesterone receptor, bcl-2, c-erbB2, p53, and P-glycoprotein between primary tumor and metastatic lymph nodes. 3) Neither wea any significant difference observed in biological behavior among the metastatic lymph nodes. CONCLUSION: This results indicate that the meaningful biological characteristic of metastatic lesion is higher histologic grade alone, and suggest that this change in histologic grade is the single, specific factor determineing the prognosis for metastatic breast cancer.