A 37-year-old man was diagnosed as malignant lymphoma infiltrating in the central nervous system with hypopituitrism and secondary glucocorticoid deficiency. In this case, plasma arginine vasopressin (AVP) increased, but glucocorticoid administration decreased plasma AVP and increased urine volume with a reduction in urinary osmolality, indicative of the presence of glucocorticoid-induced diabetes insipidus. At the terminal stage, plasma AVP did not increase in response to the withdrawal of glucocorticoid and urine volume remained decreased, suggesting the presence of masked diabetes insipidus. Autopsy showed an infiltration of lymphoid cells around the cerebral ventricles and necrosis in the hypothalamo-hypophyseal system. These findings suggested that glucocorticoid might centrally play an important role in the regulation of AVP release, and its deficiency potentiated AVP release.