There have been many reports of probable lithium-induced organic brain syndromes occurring when serum lithium levels are within or close to the therapeutic range. The authors report on five patients who developed clinical syndromes suggestive of severe neurotoxicity during lithium treatment. In all cases lithium levels were between .75 and 1.7 mEq/liter. The patients who developed neurotoxicity had markedly higher global ratings of psychotic symptomatology and anxiety in the pretoxic period than did patients who never deveoped neurotoxicity. When the acute manic state is characterized by marked psychotic symptoms and intense anxiety, it may be associated with increased vulnerability to the development of severe lithium neurotoxicity.