We report on two young patients with an encephalitic syndrome and bilateral thalamic lesions following a presumably viral or mycoplasma respiratory tract infection with the main clinical symptoms of organic psychosis in the first and a prolonged amnestic syndrome and ataxia in the second case. Four months later the patients had recovered clinically and the thalamic lesions had resolved on magnetic resonance imaging in one case and almost completely in the other. We interpret the patients' illness as rare cases of a post-infectious acute thalamic encephalitis in adults. The cases and their relationship to possible post-infectious autoimmune inflammatory or toxic pathophysiological mechanisms are discussed and a review of the literature is provided.