Non–insulin–dependent diabetes mellitus (NIDDM) affects about 5% of the world population. The disease presents a polygenic mode of inheritance, but mechanisms and genes involved in late–onset NIDDM are largely unknown. We report the association of a single heterozygous Gly to Ser missense mutation in the glucagon receptor gene with late–onset NIDDM. This mutation was highly associated with NIDDM in a pooled set of French and Sardinian patients (χ2= 14.4, P=0.0001) and showed some evidence for linkage to diabetes in 18 sibships from 9 French pedigrees (χ²=6.63, P<0.01). Receptor binding studies using cultured cells expressing the Gly40Ser mutation demonstrate that this mutation results in a receptor which binds glucagon with a three–fold lower affinity compared to the wild type receptor.