In 51 unselected breast cancer samples, a double-labeling immunocytochemical technique was utilized to observe the phenotypic expression of the nm23 gene during S-phase. The feasibility of the method was confirmed by comparison with routine evaluations for both thymidine-labeling index and nm23 (p<0.001). No correlation was found between the two parameters in the overall series or when subgroups regarding menopausal status, nodal involvement, tumor size, hormone receptor content were considered. Tumors with a higher nuclear grade showed a significant correlation only with TLI (p=0. 02). A trend for an inverse relationship between the two parameters was noted but was not statistically significant. A significant association between TLI and nm23 expression was found by Chi-square test using median values as cut-off. Our data based on morphological and in situ observations do not confirm a correlation between nm23 and cell proliferation, even if this correlation cannot be completely excluded. Multicentric trials of high power with these same techniques are still necessary to definitely establish the prognostic role of nm23 in breast cancer.