Levels of expression of the murine nm23 gene inversely correlate with metastatic potential in several rodent tumor model systems. Expression of the human nm23 homologue also is lower in human breast cancers of high metastatic potential than in breast cancers of low metastatic potential. In the present study, we examined changes in nm23 expression during colon carcinogenesis as found in 18 matched pairs of normal and neoplastic human colon tissues. We found that a 0.8-kilo-base nm23 transcript was expressed in all samples of morphologically normal colon mucosa. In 16 of 18 colon neoplasms, nm23 expression was further increased in the neoplastic, compared with the morphologically normal, colon mucosa from the same individual. Expression of nm23 was elevated over normal mucosa in 3 of 3 polyps, in 2 of 3 nonmetastatic cancers, and in 11 of the 12 cancers that were metastatic at the initial presentation. The levels of nm23 expressed were similar in the 12 metastatic colon neoplasms and in the 6 colon neoplasms of lower clinical stage. In addition, nm23 expression was maintained in culture in each of 12 cell lines initiated from human colon neoplasms and did not differ between lines established from neoplasms of high or low metastatic capability. We concluded that nm23 was expressed in normal colon mucosa. Expression of nm23 increased during early stages of colon carcinogenesis and remained increased in metastatic colon cancer. Therefore, in the colon, tissue-specific events dissociate nm23 expression from loss of tumor metastatic competence.