A 38 year old man was investigated because of impotence, gynaecomastia and galactorrhoea. Hyperprolactinaemia and hypogonadism were documented. Pituitary function was otherwise normal as was tomographic examination of the sella turcica. In the absence of direct evidence of pituitary involvement (hyperprolactinaemia can suppress gonadal function) and to exclude ectopic prolactin production, venous blood was drawn at multiple sites. The highest prolactin levels were found in the superior vena cava and above, indicating an intracranial source. At transsphenoidal hypophysectomy a microadenoma was removed; tumour cells contained typical prolactin secretory granules on electron microscopy. In the light of this report the appropriateness of dividing hyperprolactinaemia into "tumourous" and "idiopathic" subgroups on the basis of sella size must be reconsidered. Functional tests do not distinguish between the subgroups although prolactin levels tend to be higher when the sella is enlarged. Only a quantitative rather than a qualitative difference may exist between the subgroups.