The nm23 gene, which encodes nucleoside diphosphate (NDP) kinase, is proposed as a metastatic suppressor gene. Loss of heterozygosity (LOH), and the expression of the nm23 gene were examined on matched sets of primary tumors and lymph node as well as distant metastases from colorectal carcinomas. Three (15%) of the 20 informative specimens examined showed LOH for the nm23 locus. nm23 was expressed in all the carcinomas as well as in nonneoplastic colonic mucosa at the mRNA and protein levels. Most of the carcinomas expressed the nm23 transcript at higher levels than the corresponding nonneoplastic colonic mucosa. By Western blotting, the level of nm23 protein expression in the tumors showed an inverse correlation with the tumor stage. Furthermore, distant metastatic tumors in the liver and lung showed reduced nm23 immunoreactivity in comparison with their primary tumor, although nm23 immunoreactivity was the same in the primary tumors and in local lymph node metastases. These results suggest that decreased nm23 expression may be associated with distant metastatic spread.