A 12-year-old female patient presented with nonspecific symptoms including fatigue, fever, and headache persisting for approximately 4 months. Plasma ammonia was 230 μg/dL (135 μmol/L) (critical value >187 μg/dL or 110 μmol/L). The patient was referred to a pediatric emergency center for urgent evaluation of hyperammonemia. Plasma and urine amino acid testing was performed to investigate the possibility of a urea cycle disorder; however, all results were normal. Over the following 2 weeks, the patient was treated with intravenous sodium benzoate/sodium phenylacetate and L-arginine to reduce the blood ammonia concentration.