In the last few years, molecular studies on pituitary adenomas have yielded evidence supporting a primary pituitary origin of these tumors. Although the existence of genomic alterations in tumoral cells is strongly suggested by the fact that almost all pituitary adenomas are monoclonal in origin, structural genetic abnormalities such as rearrangement, deletion or mutation, which could result in transcriptional activation, have been identified in a minority of tumors. As far as the possible loss of anti-oncogenes in pituitary tumors is concerned, gene alterations have not been found in the p53 nor in the retinoblastoma gene, while loss of chromosome 11q13 sequences, which contain the deduced location of the yet uncloned MEN-1 gene, has been reported in a subset of GH-secreting adenomas. With regard to the activation of dominant oncogenes in the process of tumor formation, activating mutations of either the Gs alpha-subunit or the ras gene have been identified in a large proportion of GH-secreting adenomas and in individual particularly invasive tumours, respectively. Promoting agents such as hypothalamic neurohormones and growth factors may be required for the selective growth of genetically altered cells. In this respect, it is worth noting that receptor/postreceptor alterations occurring in pituitary tumors may cause an increased action of stimulatory neurohormones with growth promoting properties as well as defective action of inhibitory inputs.