The nm23 gene was originally identified by Steeg et al. by screening of cDNA libraries from murine melanoma cell lines of varying metastatic potential. An inverse relationship between metastatic potential and nm23 RNA and/or protein was found in four different metastatic model systems. It was proposed that nm23 may function as a suppressor gene for tumour metastasis. It has recently been found that the sequence of nm23 and NDP-kinase (NDP-K) is identical. Using an immunohistochemical technique and employing a polyclonal antibody to purified NDP-K A, we have determined NDP-K expression in a series of 197 breast carcinomas. One hundred and sixty (81.2 per cent) of these tumours were scored positive for NDP-K and 37 (18.8 per cent) scored negative. No relationship was found between NDP-K/nm23 expression and patient relapse or survival. Furthermore, no relationship was found between NDP-K/nm23 expression and a number of other prognostic factors including tumour grade, oestrogen receptor, progesterone receptor, and p53 expression. Our results contradict the hypothesis concerning the possible role of NDP-K/nm23 as a metastatic suppressor gene in human breast cancer, but further studies using antibodies specific for NDP-K/nm23 subtypes are clearly indicated.