To assess gene expression of a membrane-bound G-protein-coupled fatty acid receptor, GPR40, in the human pancreas and islet cell tumors obtained at surgery were analyzed. The mRNA level of the GPR40 gene in isolated pancreatic islets was approximately 20-fold higher than that in the pancreas, and the level was comparable to or rather higher than that of the sulfonylurea receptor 1 gene, which is known to be expressed abundantly in human pancreatic beta cells. A large amount of GPR40 mRNA was detected in tissue extracts from two cases of insulinoma, whereas the expression was undetectable in glucagonoma or gastrinoma. The present study demonstrates that GPR40 mRNA is expressed predominantly in pancreatic islets in humans and that GPR40 mRNA is expressed solely in human insulinoma among islet cell tumors. These results indicate that GPR40 is probably expressed in pancreatic beta cells in the human pancreas.