WNT2 is one of proto-oncogenes with the potential to activate the WNT - beta-catenin - TCF signaling pathway, which is most homologous to WNT2B among members of the human WNT gene family. Here, expression of WNT2 mRNA was comprehensively investigated. WNT2 mRNAs were highly expressed in fetal lung, and weakly expressed in placenta. Among 2.0-, 2.9-, 4.1-, and 6.0-kb WNT2 mRNAs, the 2.0-kb WNT2 mRNA was the major transcript in fetal lung. In 3 cases of prostate cancer and 1 case each of lung cancer and cervical cancer, WNT2 was over-expressed in non-cancerous portion as well as in primary tumor. WNT2 was up-regulated in 14 out of 18 cases of primary colorectal cancer, 4 out of 7 cases of uterus tumor, 2 out of 9 cases of breast cancer, and in 2 out of 14 cases of kidney tumor. Up-regulation of WNT2 was also detected in 4 out of 8 cases of primary gastric cancer by using expression array filter hybridization, and in 10 out of another 10 cases of primary gastric cancer by using cDNA-PCR. Frequent up-regulation of WNT2 in primary gastric cancer and colorectal cancer might play a key role in carcinogenesis through activation of the WNT - beta-catenin - TCF signaling pathway.