A 62-year-old woman presented with general malaise persisting for 2 months and narrowing of her visual field. T₁-weighted magnetic resonance (MR) imaging showed swelling of the pituitary gland and stalk, and a homogeneous isointense intra- and suprasellar mass enhanced by gadolinium. During outpatient follow up, her condition deteriorated rapidly and she developed diabetes insipidus and panhypopituitarism. T₁-weighted MR imaging demonstrated shrinkage of the lesion and disappearance of the central hypointense area indicating the cyst cavity, especially in the pituitary stalk. She underwent surgical exploration via the transsphenoidal approach under a provisional diagnosis of lymphocytic hypophysitis. Histological examination revealed ciliated columnar cells and squamous metaplasia on the inner surface of the cyst wall, with many foamy cells, infiltration by many lymphoid cells and some eosinophils, and fibrosis in the adenohypophysitis and inflammatory hypophysitis in the anterior and posterior pituitary gland. The present neuroimaging findings of cyst shrinkage associated with rapid clinical deterioration strongly support the suggestion that rupture of Rathke’s cleft cyst causes inflammatory hypophysitis.