The pathophysiological mechanism of pituitary ACTH oversecretion in Cushing's disease remains unclear. The question of whether a collection of corticotroph cells is a primary pituitary event or is driven by increased production of hypothalamic corticotropin releasing factor is still debated. Establishing whether or not there is a clonal nature of such pituitary lesions has important conceptual and practical implications. Clonal composition of corticotroph cell adenomas was determined by X chromosome inactivation analysis using a DNA probe, M27 beta, which detects a multiallelic polymorphism in 90% of females. A first digestion by PstI reveals the polymorphism. A second digestion by MspI or its methylation sensitive isoschizomer HpaII, distinguishes the active from the inactive copy. DNA was extracted from 11 corticotroph macroadenomas responsible for Cushing's disease or Nelson's syndrome. Eight of the 11 female patients were heterozygous for the locus and included in the study. Blood leukocytes were available for 5 females and were used as controls. All 8 tumors demonstrated a monoclonal pattern while the 5 leukocyte DNA were polyclonal. Ours results show that a somatic modification plays an important role in the pathogenesis of corticotroph macroadenomas allowing monoclonal expansion of a genetically aberrant cell.