Inactivation of tumor suppressor genes is now believed to play an important role in various progression stages of human cancers. To clarify the possible involvement of tumor suppressor gene inactivation in the acquisition of metastatic potential in lung and colorectal carcinoma cells, we examined various genetic alterations in both primary tumors and metastases obtained from patients with lung and colorectal carcinomas. In lung carcinoma, loss of heterozygosity on chromosomes 3p, 13q, and 17p is a common genetic alteration, and both RB and p53 genes are inactivated as a result of chromosome 13q and 17p losses. In some cases, allelic loss on chromosome 11p and amplification of myc family oncogenes occur during tumor progression. In colorectal carcinoma, p53 and DCC alterations were detected in 100% of metastases, and sequential accumulation of allelic losses on chromosomes 13q, 14q, and 18q in the process of metastasis was observed. These results indicate that a subset of tumor suppressor genes is involved in metastasis of lung and colorectal carcinomas.