PURPOSE. It is unknown to what extent lymph node metastases differ from primary tumours of breast cancer. Our aim was to investigate the similarity between primary breast tumours and the matching lymph node metastases in 59 breast cancer patients. EXPERIMENTAL DESIGN. Immunohistochemical stainings of p53, bax, bc-l2, fas and fasL were performed in primary tumours and the parallel lymph node metastases. RESULTS. When using a cut point of 10%, the concordance between primary tumours and parallel lymph node metastases in the expression of p53 was 85%, bcl-2 79%, bax 69%, fas 59% and fasL 43%. In most tumours the staining status of p53, bcl-2 and bax in the primary tumour and the corresponding lymph node did not change more than 20%. However, these variables could fluctuate in both directions. In 15-25% of the cases, nodal expression was more than 20% lower than in the primary tumours, while in 10-17% of the cases, nodal expression was more than 20% higher than in the primary tumours. In half of the tumours, fas status did not change. Most fasL positive tumours lost positivity in the lymph node metastases or showed positively staining cancer cells only in the peripheral region of the node. A phenotype analysis of combined information of tumour fas/tumour fasL/nodal fas/nodal fasL expression (+/ - ) was assessed. The most frequently observed phenotype was tumour fas - /tumour fasL + /nodal fas - /nodal fasL- (22% of the tumours), although almost all combinations were seen. CONCLUSIONS. The expression of p53, bax, bcl - 2, fas and fasL is not maintained in the matching lymph node metastases of breast cancer. Large studies comparing the expression of relevant tumour biology factors in primary tumours and parallel lymph node metastases and their impact on therapy outcome, especially in the adjuvant setting, are warranted.