Downregulation of nm-23 antimetastasis gene has been associated with disease progression in some human tumors. NPD kinase A is the product of the H1 isotype of the nm23 gene and its value as a marker of metastatic potential is well worth investigating. The expression of the nm23-H1 gene peptide was immunohistochemically evaluated in 191 primary mammary cancer tissues. A three-step immunoperoxidase staining procedure was performed and any association of our results with several classical clinicopathologic indicators, including hormonal status and c-erbB-2 oncoprotein membrane immunoexpression, was examined. NDP kinase A-positive cytoplasmic immunolabeling was noticed in 64% of all specimens (123/191) which frequently demonstrated positive progesterone receptor (PgR) status (p = 0.001) and were furthermore characterized by high PgR immunoreactivity rates. This association was significant by both univariate and multivariate statistical analysis. The double nm23-H1 (+)/PgR(+) phenotype was more frequently detected than any other combined phenotype of these markers. The nm23-H1 gene peptide was generally detected in a remarkable proportion of malignant cells, either in the invasive or the intraductal tumor components. Notably, large-cell ductal carcinomas in situ were characterized by lower nm23-H1 immunoreactivity rates when compared with other in situ cancer types. Quantitatively increased nm23-H1 immunopositive staining was more frequently observed in special histologic types of infiltrating cancers, in high nuclear grade tumors, as well as in carcinomas with high PgR levels (p = 0.05). The nm23-H1 (-)/c-erbB-2(+) phenotype was more often detected in the cancers of this study than the nm23-H1(+)/c-erbB-2(+) one. The former phenotype was correlated to postmenopausal ages as well as to extensive axillary nodal involvement by univariate statistical analysis. It is noteworthy that nm23-H1(-) status, on its own, was not statistically associated either with the presence or with a high number of involved lymph nodes. On the contrary, nm23-H1 immunopositivity was, paradoxically, more frequently observed in tumors of relatively increased TN tumor stage. Tumor progression is thus more likely to depend on the c-erbB-2 gene's overexpression. Possibly, any favorable outcome in nm23-H1(+) cases might be due to the fact that they also express PgR, which is a marker of a more functionally differentiated phenotype.