We had performed a global analysis of the gene expression of gastric cancer cell lines established from malignant ascites to identify the novel markers for the detection of micro-metastasis in peritoneal cavity. One of the up-regulated genes is Reg IV, which is a member of the Reg gene family belonging to calcium dependent lectin (C-type lectin) gene superfamily. But the role of Reg IV in peritoneal dissemination is still unclear. We have examined the potential of Reg IV as a novel marker for the detection of peritoneal micro-metastases of gastric cancer. Reg IV expression was examined by quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Mean Reg IV mRNA expression levels in surgically resected specimens (n = 41) were more than 20-times higher than those in normal mucosa from those patients. Furthermore, Reg IV mRNA expression level in the peritoneal wash was strongly higher in peritoneal metastasis compared to those without peritoneal metastasis. These results suggest that Reg IV may be involved in peritoneal dissemination of gastric cancers and Reg IV would be a potential novel marker for peritoneal dissemination of gastric cancers.