We studied 25-year-old HLA- and blood group-identical male twins who had multiple endocrine neoplasia, type I (MEN I). At the time of initial examination, one twin (case 1) had epigastric pain and diarrhea; he was cushingoid in appearance. Further evaluation revealed primary hyperparathyroidism, Zollinger-Ellison syndrome, Cushing's disease, and hyperprolactinemia. Immunostaining of a resected pituitary specimen demonstrated both prolactin and, to a lesser extent, growth hormone reactivity. The nontumorous adenohypophysis showed corticotropic hyperplasia. In contrast, the other twin (case 2) was asymptomatic. He had only primary hyperparathyroidism and hyperprolactinemia. An invasive pituitary adenoma was resected and showed similar proportions of cells with immunoreactive prolactin and those with growth hormone; no nontumorous gland was available for study. Apparently, factors other than heredity may play a role in the expression of MEN I.