42 women with amenorrhoea and hyperprolactinaemia had trans-sphenoidal surgery and resection of histologically verified pituitary adenomas. 74% of these patients developed amenorrhoea and/or galactorrhoea in immediate association with the use or discontinuation of oral contraceptives or post partum. There was enough adenomatous tissue for immunocytochemical studies in 35 specimens and specific localisation of prolactin was possible in 31. There is evidence that about 10% of the population have small pituitary tumours, and the majority of these tumours, though asymptomatic, are potentially prolactin-secreting. It is suggested that oestrogens, which are known to modulate prolactin secretion in normal human beings and in animals, can induce the growth and expression of otherwise silent pituitary lesions and that this should be considered a risk of oral-contraceptive use.