[en] Mutations of the aryl hydrocarbon receptor interacting protein (AIP) gene are associated with pituitary adenomas that usually occur as familial isolated pituitary adenomas (FIPA). Detailed pathological and tumor genetic data on AIP mutation-related pituitary adenomas are scarce. Non-identical twin females presented as adolescents to the emergency department with severe progressive headaches caused by large pituitary macroadenomas requiring emergency neurosurgery; one patient had incipient pituitary apoplexy. Post-surgically the patients were found to have silent somatotrope adenomas on pathological examination. Furthermore, the light microscopic, immunohistochemical and electron microscopic studies demonstrated tumors of virtually identical characteristics. The adenomas were accompanied by multiple areas of pituitary hyperplasia, which stained positively for growth hormone, indicating somatotrope hyperplasia. Genetic analyses of the FIPA kindred revealed a novel E216X mutation of the AIP gene, which was present in both affected patients and the unaffected father. Molecular analysis of surgical specimens revealed loss of heterozygosity (LOH) in the adenoma, but showed that LOH was not present in the hyperplasic pituitary tissue from either patient. AIP immunostaining confirmed normal staining in the hyperplastic tissue and decreased staining in the adenoma in the tumors from both patients. These results demonstrate that patients with AIP germline mutation can present with clinically silent somatotroph pituitary adenomas. The finding of somatotrope hyperplasia unaccompanied by AIP LOH, suggests that LOH at the AIP locus may be a late event in the progression from hyperplastic to adenomatous tissue.