Human X-box binding protein 1 (XBP-1) is a transcription factor essential for hepatocyte growth, as well as for plasma cell differentiation. Recently, overexpression of XBP-1 has been reported in breast cancer including non-invasive carcinomas, and was suggested to play an important role in breast carcinogenesis. To investigate the involvement of XBP-1 in colorectal tumorigenecity, the expression of XBP-1 was examined in four colon cancer cell lines, six colorectal polyps and five colorectal carcinomas. The study population consisted of eleven patients who had undergone resection for colorectal cancer or adenoma from 2000 to 2002. Four colon cancer cell lines, DLD1, SW480, HCT15 and WiDr, were also analyzed for expression of XBP-1. Reverse transcription-polymerase chain reaction was performed using eleven primary colon tumors. XBP-1 expression was then investigated using an immunohistochemical method for archived paraffin-embedded sections. The XBP-1 gene was overexpressed in four cases out of five primary colorectal carcinomas and in four cases out of six colorectal adenomas. Also all four cancer cell lines expressed XBP-1 mRNA. Immunohistochemical staining demonstrated that XBP-1 protein was strongly stained in the cytoplasms of cancer cells, whereas it was unreactive in the normal colon epithelial cells and stromal cells. These data indicate that increased expression of XBP-1 gene may play some role in human colon carcinogenesis through impairment of cell differentiation regulation.