An 81-year-old man was admitted to our hospital because of general fatigue. Hormonal examination showed that he had panhypopituitarism and central diabetes insipidus. MRI imaging revealed the presence of large cystic mass with suprasellar extension in his hypothalamo-pituitary region. Interestingly, the cystic mass shrank following the start of glucocorticoid replacement, and since then relatively high doses of cortisol administration were needed to prevent the re-enlargement of cystic size. Because of the concern over possible side effects of supraphysiological doses of glucocorticoid replacement, surgical treatment was eventually carried out, confirming the pathological feature of Rathke's cleft cyst. The present case suggests that the inflammatory nature of Rathke's cleft cyst may explain the observed short-term size changes in response to glucocorticoid administration.