Activating mutations of Gs alpha (gsp) and Gi2 alpha (gip) have been described in various endocrine neoplastic conditions. The objective of this study was to assess the prevalence of gsp and gip mutations in human adrenocorticotrophin hormone-secreting pituitary adenomas. Adrenocorticotrophin hormone production and secretion by pituitary corticotroph cells is under stimulatory control by corticotrophin-releasing factor, acting via the production of cyclic AMP. Interference with this regulatory pathway as a result of G-protein dysfunction could lead to disordered corticotroph cell function and growth. We have studied 32 corticotroph adenomas for the presence of gsp and gip mutations using site-directed oligonucleotide hybridization of polymerase chain reaction-amplified DNA. G-protein gene mutations were identified in three (9%) tumours: gsp mutations were demonstrated in two tumours at codon 227, and a gip mutation was identified in one tumour at codon 179. We did not observe a correlation between tumour phenotype and the presence of G-protein gene mutations. We conclude that G-protein gene mutations are an uncommon abnormality in corticotroph adenomas.