A case of hypothalamic encephalitis proved by postmortem examination is reported. The patient was a three-year-old Chinese boy who had complained of obesity for one year and fever for half a year. Three days after admission to a hospital, the patient suddenly died of allergic reaction to intramuscular penicillin, without additional complaints. Autopsy revealed severe bilateral outfall of neurons in the ventromedian hypothalamic nuclei with diffuse hyperplasia of astrocytes and some glial nodules consisting chiefly of oligodendroglias and perivascular cuffing. Similar changes were noticed in hypothalamus at the level of optic chiasm, but the posterior hypothalamic nuclei and lateral hypothalamic nuclei were intact. This article provides significant data for the study of hypothalamic function.