Lymphocytic hypophysitis was first recognized postmortem, then by biopsy, but detection of antipituitary autoantibodies by immunofluorescence has proved unsatisfactory. Immunoblotting has the dual advantages of increased specificity and identification of the mol wt of autoantigens. Sera from 115 patients and 52 normal subjects were immunoblotted against human autopsy pituitary cytosolic proteins. Among the neurosurgical cohort (30), 10 patients had biopsy-proven lymphocytic hypophysitis, and 20 had hypopituitarism secondary to tumor. There were 22 cases with suspected hypophysitis; 47 with either Hashimoto's, Graves', or Addison's diseases; and 15 with rheumatoid arthritis. Antipituitary autoantibodies reactive to a 49-kDa pituitary cytosolic protein were found in 70% of biopsy-proven lymphocytic hypophysitis, 55% of suspected hypophysitis, 42% of Addison's disease, 20% of pituitary tumors, 15% of patients with thyroid autoimmunity, 13% of rheumatoid arthritis patients, and 9.8% of normal subjects. Reactivity to a 40-kDa cytosolic protein was also found in 50% of patients with biopsy-proven disease. These 49- and 40-kDa autoantigens are conserved across species and are not exclusive to pituitary tissue. Immunoblotting has demonstrated antipituitary autoantibodies to 49- and 40-kDa cytosolic proteins in biopsy-proven cases of lymphocytic hypophysitis.