Among 92 surgically removed pituitary adenomas immunostained for prolactin and growth hormone, 70 showed positive staining for prolactin. The majority of these (54) was associated with hyperprolactinemia leading to amenorrhea (and often galactorrhea) in women of reproductive age. Similar tumors, asymptomatic or conducive to disturbances of sexual function, were found in six hyperprolactinemic men. Among nine acromegalics, seven had immunostained lactotrophs associated with the somatotrophic adenomas cells, but only two of these had hyperprolactinemia. In all of the remaining tumors that had at least some immunoreactive lactotrophs, mild hyperprolactinemia had been present This indicates that immunostaining of pituitary tumors for prolactin correlated well with elevated plasma prolactin levels, except in the case of mixed somatolactotrophic adenomas. The patterns of distribution of immunoreactive prolactin in adenoma cells are illustrated. Since only some of the prolactin-producing adenomas stained with carmoisine--a dye that has been suggested as a marker for prolactin cells--immunocytochemistry is the method of choice for the identification of prolactin-secreting adenomas.