Lymphocytic hypophysitis (adenohypophysitis) is a rare lymphocytic infiltration of the pituitary gland that usually occurs during pregnancy. Because of its rarity, it has seldom been diagnosed preoperatively, and no trials of therapeutic treatment have been reported to date. We describe a 29-year-old woman with a pituitary mass and visual-field defects during pregnancy. The patient's pituitary profile revealed an abnormal thyroid axis and relatively low prolactin for her stage of pregnancy. This finding suggested adenohypophysitis, and the patient was given a trial course of dexamethasone. The progression of the visual-field defects, however, indicated that the steroids, both in dosage and duration, were not effective. Thus, the patient underwent a partial hypophysectomy for decompression. The pathology report confirmed adenohypophysitis, and steroids were continued for the remainder of the pregnancy, with slow resolution of the visual-field defects to normal. This report is the first case of adenohypophysitis where the diagnosis was suspected preoperatively, and a trial course of steroids was given. The steroids at the dosage and duration used were not effective. Further evaluation of the use of steroids in this disease is warranted.