The nm23 gene was originally identified by differential hybridization of metastatic murine melanoma cell lines. Some experimental studies demonstrated a significantly reduced metastatic potential of melanoma cell lines transfected with the nm23 gene. In this study, we clarified the relationship between lymph node status and nm23 immunoreactivity, as well as Ki-67 labeling index (LI), of human breast cancer. Of the 44 breast invasive ductal carcinomas, nm23-diffusely positive expression [nm23(+)] was detected in 17 (38.6%), and focally positive/negative expression [nm23(+/-/-)] in 27 (61.4%) cases. Lymph node metastasis was found at a significantly higher incidence in the nm23(+/-/-) cases (18/27, 66.7%) than in the nm23(+) cases (4/17, 23.5%) (p>0.001). In the lymph node metastasis-positive cases, mean LI of Ki-67 cells was 20.9% at the center of the tumors and 24.0% at the advanced margins. In the lymph node metastasis-negative cases, mean LI of Ki-67 cells was 12.4% at the center of the tumors and 27.2% at the advanced margins. Decrease of nm23 expression, but not Ki-67 LI, was significantly correlated with lymph node metastasis of breast invasive ductal carcinoma.