Pituitary tumours are characterized by a series of phenotypic abnormalities, but the molecular nature of the underlying defects has proved peculiarly intractable. Oncogenes and tumour suppressor genes involved in other tumours do not appear to play a major role in the pathogenesis of pituitary tumours. In addition, germline genetic disorders in which pituitary tumours are a common feature have not shed much light on the more common sporadic tumour. A number of defects in specific feedback regulation in the secretory tumours have been identified, but it is presently unclear as to what extent these are a consequence of the tumour, possibly enhancing its growth or survival, rather than the cause. However, recent studies on the cell cycle have demonstrated significant abnormalities that have been traced to a cytoplasmic kinase which appears to be abnormally expressed in the majority of pituitary adenomas, and we are beginning to see a possible unifying abnormality.