: An 87-year-old man was admitted with fatigue, anorexia, vomiting, urinary incontinence, and a depressive state. His consciousness was evaluated as a 13 on the Glasgow Coma Scale (E4V3M6), and he had a body temperature of 36.4°C, a blood pressure of 91/60 mmHg, and a heart rate of 88 beats/min. General laboratory data were unremarkable except for a mildly elevated serum creatinine level. The plasma levels of growth hormone, luteinizing hormone, and follicle stimulating hormone were depressed. On the other hand, the prolactin level was elevated, and the corticotropin, cortisol, and thyrotropin levels were within the reference ranges. Cranial magnetic resonance imaging (MRI) revealed the marked swelling of the pituitary gland and the infundibular stalk, and the serum immunoglobulin G4 (IgG4) level was elevated (2.85 g/L; reference range, 0.048-1.05 g/L). Accordingly, a diagnosis of IgG4-related autoimmune hypophysitis (AH) was made. The patient responded well to glucocorticoid therapy, but the presence of diabetes insipidus was revealed and was subsequently controlled using desamino-D-arginine vasopressin (DDAVP). To our surprise, an empty sella was apparent on an MRI examination performed on Day 12. The patient's serum IgG4 level had decreased in a log-linear manner with a half-life of 30 days, which was comparable to the half-life of IgG4 in control subjects (21 days). At a 16-month follow-up examination, no substantial changes in the morphology or function of the pituitary gland were noted. In conclusion, an empty sella developed within 12 days after the clinical onset of AH in the present case, suggesting that an empty sella may be the direct outcome of AH. The conversion of AH to an empty sella was associated with an immediate shutdown of IgG4 overproduction.