We previously performed a global analysis of the gene expression of gastric cancer cell lines established from peritoneal dissemination (SNU-5, SNU-16, SNU-719, KATO-III and GT3TKB) with the cDNA microarray method 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. We have examined Reg IV potential as a novel marker for the detection of peritoneal micro-metastases of gastric cancer. Reg IV expression was examined in five gastric cancer cell lines established from peritoneal dissemination and compared with myeloid leukemia cell (HL60), methothelial cell lines Met5A and the other gastric cell line established from primary tumor (SNU-1) by quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Reg IV was highly overexpressed in 4 gastric cancer cell lines established from peritoneal dissemination, but weakly expressed in other cell lines. According to Reg IV mRNA expression levels in surgically resected specimens, the quantity of Reg IV correlated with wall penetration. Furthermore, Reg IV mRNA expression level in the peritoneal wash from 35 gastric cancer patients was also prone to correlation with wall penetration. These results suggest that Reg IV may be involved in peritoneal dissemination of gastric cancers and Reg IV may be a potential novel marker for peritoneal dissemination of gastric cancers.