To determine the relationship between the mutation of the PTEN gene and genomic instability in human colorectal cancer, we screened the PTEN gene in 32 colorectal cancers (eight cell lines and 24 tissues) displaying microsatellite instability (MSI) and 32 colorectal cancers (six cell lines and 26 tissues) displaying microsatellite stability (MSS). Of 64 samples, six frameshift mutations were identified in six MSI colorectal cancers (two cell lines and four tumor tissues) but not in MSS colorectal cancers. Of the six MSI colorectal cancers containing PTEN mutation, two case (one cell line and one tumor tissue) showed biallelic mutation of PTEN. Furthermore, to compare the frequency of PTEN mutation with that of other MSI target genes, the mononucleotide repeat sequences of TGF-betaRII, BAX, IGFIIR, hMSH6 and hMSH3 were analyzed. The frequencies of frameshift mutation were revealed 78% in TGF-betaRII, 56% in BAX, 46.9% in hMSH3, 37.5% in hMSH6, 21.9% in IGFIIR, and 18.8% in PTEN. These results suggest that the PTEN gene is a target of genomic instability in MSI colorectal tumorigenesis.