A case of carbamazepine-induced hyperammonemia is presented.A 26-year-old man with bipolar disorder, seizures, and mild mental retardation secondary to a traumatic brain injury began treatment with carbamazepine for aggression and seizure control. After three weeks of carbamazepine therapy, the patient arrived at the emergency department (ED) with severe agitation and aggressive behavior. His oral medications included topiramate, carbamazepine, olanzapine, quetiapine, guanfacine, and desmopressin acetate. The patient's medications had been stable for at least six months except for the addition of carbamazepine one month before his arrival at the ED. Upon admission, the patient's vital signs were found to be within normal limits, as were his liver profile results, complete blood count, thyroid-stimulating-hormone level, and serum chemistry panel. His serum carbamazepine concentration was 3.9 microg/mL (reference range, 4-12 microg/mL), and his serum ammonia concentration was 127 microg/dL (reference range, 19-60 microg/dL). Carbamazepine was discontinued upon admission, and the patient was treated with oral lactulose. Since carbamazepine was discontinued and had been prescribed for bipolar disorder, his olanzapine dosage was increased, and trazodone was added at bedtime for insomnia. Of note, the patient had been on carbamazepine therapy one year earlier and had experienced the same adverse event. He had also developed elevated serum ammonia levels while on valproic acid. The patient's serum ammonia level returned to normal by hospital day 4, and he was discharged to his group home.A 26-year-old man with bipolar disorder developed hyperammonemia three weeks after initiating carbamazepine therapy.