Lithium-induced delirium occurring in geriatric patients with serum lithium levels that are within the "therapeutic" range (less than 1.5 mEq/L) has been described in the literature. We present a case that illustrates three major issues regarding this syndrome: (1) differentiating lithium-induced delirium from a recurrence of a chronic psychiatric disorder; (2) the use of the electroencephalogram in supporting this diagnosis; and (3) factors that may increase a patient's vulnerability to delirium while on lithium. A brief review of the most relevant literature is then presented. We conclude that lithium-induced neurotoxicity should be suspected in any patient receiving lithium who develops delirium, regardless of the serum level, and that immediate discontinuation of the medication be considered.