Idiopathic hypothalamic dysfunction is a rare disorder associated with adipsia, obesity and other symptoms such as central hypoventilation without any hypothalamic structural lesion. We report the case of a 3-year-old boy who died suddenly due to central hypoventilation. His obesity index increased from 0% to 75% during the 7 months prior to 3 yr 0 mo. During this clinical course, adipsia, hyperthermia, sudoresis, mild central hypoventilation, blepharoptosia, a change of character, hypernatremia, and hypothalamic hypopituitarism were observed. No treatment was given for hypoventilation. He died from sudden respiratory arrest because of central hypoventilation at 3 yr 8 mo. MRI examination was performed twice at 3 yr 3 mo and 3 yr 4 mo and revealed neither specific lesion nor invisible pituitary stalk. At autopsy, local inflammation and gliosis of the hypothalamus were present. So far at least 14 patients with idiopathic hypothalamic dysfunction have been reported. Eight cases including ours had central hypoventilation. Six out of the eight had poor respiratory prognosis, and central hypoventilation in hypothalamic dysfunction could be potentially fatal. In conclusion the patients with hypothalamic dysfunction should be treated intensively if they are suspected of having central hypoventilation.