Hypogonadism is a component of the polyglandular autoimmune syndromes and usually results from primary gonadal failure. Isolated gonadotropin deficiency is a disorder of prepubertal onset that usually results from a failure of hypothalamic secretion of gonadotropin-releasing hormone (GnRH). We describe here two men with polyglandular autoimmune syndrome and isolated gonadotropin deficiency acquired after puberty. Plasma levels of luteinizing hormone and follicle-stimulating hormone in response to bolus doses of GnRH and to pulsatile GnRH injections (25 ng per kilogram of body weight intravenously every two hours) over a four-day period were subnormal. Pituitary secretion of thyroid-stimulating hormone, prolactin, growth hormone, and ACTH was not impaired. These data indicate that isolated gonadotropin deficiency may result from a selective pituitary gonadotrope failure. In addition, they suggest that autoimmune hypophysitis may be an integral part of the polyglandular autoimmune syndrome and can be selective, involving only one type of pituitary cells.