OBJECTIVE: Eight cellular homologs of the inhibitors-of-apoptosis proteins (IAP) have been identified in humans and of them, the X-linked IAP (XIAP) is the most potent. XIAP-associated factor 1 (XAF1) is a newly discovered XIAP-binding protein that negatively regulates the caspase-inhibiting activity of XIAP. It is either not expressed or present at extremely low levels in many cancer cell lines. The aims of the present study were: (i) to investigate the expression of XAF1 in human colorectal cancers (CRC) both in vitro and in vivo, and (ii) to evaluate the possibility of XAF1 as a new tumor marker. METHODS: The expression of XAF1 in four human colon cancer cell lines (Colo205, Colo320, SW1116, LoVo) and in samples from 70 patients with CRC was analyzed by reverse transcriptase-polymerase chain reaction. XAF1 concentrations were also detected in the peripheral circulation of the 70 patients, as well as three traditional circulating cancer-associated antigens. RESULTS: A low concentration of XAF1 mRNA was detectable in the three colon cancer cell lines other than Colo205, which showed the strongest expression of XAF1. The expression of XAF1 in tissue was relatively lower in primary CRC compared with a relatively higher level in benign colorectal tumors (P < 0.01). Although the XAF1 expression in circulation of those with CRC was also lower than in those with benign tumors, there was no statistical significance (P > 0.05). CONCLUSIONS: The present results suggest that the low expression of XAF1 in tumor tissue coincides with a similar level in the peripheral circulation, which contributes at least part to the malignant behavior of CRC. Integrating the XAF1 relative expression value with the other three traditional tumor biomarkers created a four-parameter assay that significantly improved the rate of diagnosis of CRC.