This report describes the results of a long term prospective study of 30 women with hyperprolactinemia who were not treated and who underwent yearly clinical, hormonal, and radiographic evaluation for an average of 5.2 yr (range 3-7 yr). At entry into the study 18 women had amenorrhea, 8 had oligomenorrhea, and 4 had regular menstrual periods. The initial mean serum PRL levels did not differ in women grouped according to menstrual function. Nine women (35%) had improvement in clinical symptoms. Serum PRL decreased, and menstrual periods normalized more often in those who initially had oligomenorrhea or regular menstrual periods. In most amenorrheic women serum PRL levels did not decline, and menstrual symptoms did not improve. Six of 30 women had an increase in serum PRL, 14 had no change, and 10 had a decrease, in 6 of whom serum PRL was normal at the last observation. Twenty-seven women had serial radiographic studies. Four (15%) of the 13 women with initially abnormal radiographic findings had normal studies later, 2 had tumor progression, and 7 no change. Four of 14 women who had normal radiographic studies initially developed radiographic evidence of a pituitary tumor, although the radiographic changes were minimal, and no patient developed a macroadenoma or pituitary hypofunction. Increases or decreases in serum PRL did not accurately predict changes in tumor size. Prior estrogen use and previous pregnancies did not increase the likelihood of tumor appearance or enhance tumor growth. The clinical presentation of the patient was an important factor in predicting which patients had a decline in serum PRL and resolution of symptoms. We conclude that patients with hyperprolactinemia are unlikely to have progression of their disease and may, in fact, have clinical and radiographic improvement.