The nm23 gene is a potential metastasis suppressor gene originally identified using a murine melanoma cell line. The nm23 proteins are nucleoside diphosphate (NDP) kinases, and there are two isotypes in human and other mammalian cells. Immunohistochemical analysis with anti-rat NDP kinase (NDP kinase alpha, NDP kinase beta) monoclonal antibodies was performed in deparaffinized sections of 56 specimens of oral squamous cell carcinoma. Detection of these NDP kinases in tissue samples was correlated with clinical and histopathological factors. The NDP kinase A (nm23 H1 protein)-positive group was apt to have a lower frequency of neck metastasis than the negative one (p < 0.01). Consequently, it is suggested that NDP kinase A might be a metastasis suppressor factor that may be useful for predicting tumor metastases in patients with oral squamous cell carcinoma.