Guanylin is a mammalian peptide ligand that binds to the enterocyte receptor guanylyl cyclase C and mediates Cl- and HCO3- efflux via the cystic fibrosis transmembrane conductance regulator. To identify the regional localization of guanylin mRNA in the human intestine, we performed in situ hybridization using a guanylin-specific riboprobe. The pattern of guanylin mRNA distribution is complex and includes all epithelial lineages at various points along the duodenal-to-colonic axis. Guanylin mRNA expression is most prominent in the distal small intestine and colon. In the normal colon, guanylin mRNA is robustly expressed in superficial epithelial cells; in colorectal adenocarcinoma, however, guanylin mRNA expression is absent. Guanylin mRNA is detectable in several intestinal tumor cell lines, although at much lower levels than those seen in the human intestine. The pattern of guanylin expression is consistent with the possibility of region-specific functions for guanylin within the human intestine. Furthermore, the diminished expression of guanylin mRNA in adenocarcinoma of the colon and in colon cancer cell lines, along with the chromosomal localization of guanylin to the tumor modifier region 1p34-35, raises the possibility that loss of guanylin activity leads to or is a result of adenocarcinoma formation.