<i>Objective:</i> To report a case of infundibuloneurohypophysitis treated with steroid. <i>Clinical Presentation:</i> A 65-year-old woman who was well until 4 weeks before admission and was not taking any medication presented with acute development of polydipsia and polyuria. Urinary volume was increased to 4,500 ml/day. She showed elevated serum osmolality and low urine osmolality, together with shortage of antidiuretic hormone. Magnetic resonance imaging (MRI) of the pituitary revealed marked nodular thickening of the neurohypophysis. Endocrinologically, anterior pituitary function appeared normal. Based on these examinations, she was diagnosed as having central diabetes insipidus due to lymphocytic infundibuloneurohypophysitis. <i>Intervention: </i>Prednisolone (1 mg/kg/day, p.o.) and <i>D</i>-deaminovasopressin (5 μg/day, intranasal) were commenced. Ten days after the administration of the agents, MRI showed a dramatic improvement in the thickening of the neurohypophysis. Ten weeks later, abnormalities found in earlier MRI had disappeared. The drugs were withdrawn gradually, and diabetes insipidus ceased 25 weeks later. Recurrence was not seen in the subsequent MRI, and the function of the posterior pituitary gland was completely normalized even 7 years after discontinuation of treatments. <i>Conclusion:</i> This case shows that noninvasive diagnosis and appropriate steroid administration can effectively cure lymphocytic infundibuloneurohypophysitis; it is recommended with long-term follow-up.