AIDS dementia complex is one of the specific infectious dementias, which is rarely seen in senescent (>75 years of age) subjects. Hashimoto encephalopathy has been described as the cause of several neurological and psychiatric syndromes including dementia. The proposed pathophysiological mechanism is an autoimmune reaction to shared, thyroid gland and CNS epitopes with subsequent cerebral dysfunction. We report here the first case of a patient who fulfils both, the criteria for AIDS dementia complex and Hashimoto encephalopathy, yet being unresponsive to steroid therapy. Diagnostic and therapeutic implications are discussed.