One hundred patients, 84 females and 16 males, with presumed prolactin-producing pituitary adenomas were treated with transsphenoidal microsurgery. Criteria for success of the operation were return of serum prolactin values to normal and reversal of the endocrine effects of hyperprolactinemia, eradication of the adenoma, and preservation of anterior pituitary function. Of the 84 females, 54 had a microadenoma (a noninvasive adenoma 10 mm or less in diameter), and of the 32 with preoperative values of serum prolactin of less than 100 ng/ml, 28 (88%) had a successful operative result, whereas 11 of 22 (50%) with preoperative values of 100 ng/ml or more had a successful result. Five of 10 patients (50%) with a diffuse adenoma (a noninvasive adenoma more than 10 mm in diameter) and 5 of 20 (25%) with an invasive adenoma had a successful result. Of the 16 males, 3 of 7 (43%) with diffuse adenoma and 3 of 16 (19%) with invasive adenoma had a successful result. There were no males with microadenoma. The largest success rate (88%) was among female patients with microadenoma and preoperative serum prolactin values of less than 100 ng/ml.