Growth hormone (GH) releasing hormone (GHRH) is a hypothalamic factor that stimulates GH secretion. It acts by activating a seven transmembrane domain G protein-coupled receptor of 423 amino acids expressed by the somatotroph cells of the pituitary gland (GHRH receptor, GHRH-R). Familial isolated growth hormone deficiency (IGHD) can be caused by mutations in the GHRH-R gene both in humans and mice. We have described six disease-causing missense mutations in this gene in IGHD patients (H137L, L144H, A176V, A222E, F242C, K329E). These mutations are inherited as autosomal recessive traits, and cause impairment of the receptor to transmit GHRH signalling. The aim of this study is to investigate the mechanisms through which these mutations cause receptor malfunction. To this end, we transiently expressed each mutated receptor into Chinese hamster ovary cells. Cells expressing each of the mutated receptors did not show an increase in intracellular cyclic AMP in response to GHRH. Immunoprecipitation and immunofluorescence studies indicated that the amino acid changes do not cause protein degradation, and do not alter the proper insertion of the receptor into the cell membrane. Binding studies with human 125I-GHRH showed that the lack of response to GHRH is due to inability of all the mutated receptors to bind the ligand. These studies demonstrate that abnormal ligand binding is a common mechanism by which naturally occurring missense mutation alter GHRH-R function.