Expression of a recently identified murine gene, nm23, has been previously proposed to be inversely correlated to tumor metastatic potential in rodent model systems. The present study was designed to investigate whether nm23 RNA was detectable in human tumor tissue, and if it was differentially expressed. nm23 RNA levels in 27 human primary infiltrating ductal breast carcinomas were determined by using Northern blots or in situ hybridization. These data were compared to traditional histopathological indicators of metastatic potential, including the number of involved (tumor bearing) lymph nodes, grade of differentiation, and hormone receptor status. A striking consistency was observed in all tumors from patients with involved lymph nodes. Using Northern blot or in situ hybridizations, all of these tumors expressed low levels of nm23 RNA. Quantitative in situ hybridization on tumors from patients with 0 involved lymph nodes identified two groups (a) approximately 75% contained high nm23 RNA levels, and (b) 25% contained significantly (α = 0.05) lower nm23 RNA levels. Low nm23 RNA levels in the 0 involved lymph node tumors were accompanied by two additional histopathological indicators of high metastatic potential, low nuclear and cytoplasmic estrogen receptor content, and poorly differentiated histological grade. In contrast, none of the high nm23 RNA level tumors were both receptor negative and poorly differentiated. We conclude that nm23 RNA levels are differentially expressed in human breast tumors, and that low nm23 RNA levels are associated with histopathological indication of high metastatic potential. Short term (median follow-up of 16 months) clinical course data were consistent with nm23 RNA levels, in that 2 of 11 low nm23 RNA content patients (including one from the 0 involved lymph node group) developed metastases, while none of the high nm23 RNA patients have experienced recurrent disease.