The frequency of detection of serum antibodies against pituitary cells was determined in 32 patients with the primary empty sella syndrome. Antibodies reacting with corticotropin-secreting mouse AtT20 and PRL-secreting rat GH3 cells were found in 24 (75%) and 15 (47%), respectively, of the 32 patients; 14 patients (44%) had antibodies reacting with both cell lines. In patients with pituitary adenomas, the prevalence of antipituitary antibodies was significantly lower than in those with the empty sella syndrome; 1 of 9 acromegalic patients had antibodies reacting with GH3 cells, and 2 of 9 prolactinoma patients and 1 of 7 patients with nonfunctioning adenomas had antibodies reacting with both AtT20 and GH3 cells. Among 6 patients with idiopathic diabetes insipidus, 1 patient had antibodies reacting with AtT20 and GH3 cells, and 2 patients had antibodies reacting with either AtT20 or GH3 cells. None of 5 patients with established autoimmune diseases (3 with systemic lupus erythematosus and 2 with autoimmune adrenal failure) had antipituitary antibodies in their serum. These results suggest that pituitary antibodies may be related to the development of pituitary atrophy and the primary empty sella syndrome, and that the test may be clinically useful as a screening test for the empty sella syndrome.