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Reye syndrome is seldom considered in the older child presenting with acute encephalopathy. The paucity of literature on Reye syndrome in the past 30 years highlights the importance of sharing new observations of this less-commonly considered disease. This case details an atypical presentation of Reye syndrome in an adolescent girl whose course was unexpectedly complicated by dialysis disequilibrium syndrome, occurring as a result of rapid metabolic shifts in the setting of urgent hemodialysis. A 14-year-old, previously healthy girl born to non-consanguineous Pakistani parents presented with acute-onset encephalopathy characterized by aggressive behaviour and altered level of consciousness. This was preceded by frequent vomiting and weeks of disordered eating. Laboratory findings included high anion-gap [30 (normal range, <14)] metabolic acidosis [pH 7.23 (normal range, 7.35-7.45), bicarbonate 10 mmol/L (normal range, 22-29 mmol/L)] with markedly elevated peak levels of lactate [13.6 mmol/L (normal range, 0.5-2.2 mmol/L)] and ammonia [573 μmol/L (normal range, 11-48 μmol/L)]. Emergent hemodialysis was undertaken for management of severe hyperammonemia, with subsequent worsening of cerebral edema, elevated intracranial pressure, and consequent uncal and tonsillar herniation, in keeping with the development of dialysis disequilibrium syndrome. The patient was ultimately diagnosed with Reye syndrome after extensive biochemical and molecular testing including exome sequencing were non-contributory. This diagnosis would be consistent with her poor nutritional state, depleted carnitine levels, mild liver dysfunction and hyperammonemia. Two weeks of intensive care were required. The patient was ultimately discharged to a rehabilitation facility with ongoing fatigue and regression in motor and cognitive domains. Hyperammonemic encephalopathy should prompt consideration for Reye syndrome. Hyperammonemia is a medical emergency; the extremely time-sensitive nature of hyperammonemia management mandates that ammonia level testing be undertaken for any unexplained encephalopathy. Severe elevations can result in neurological impairment, cerebral edema, and are acutely life-threatening. Hemodialysis may be indicated for levels >150 μmol/L; the urgency of ammonia clearance must be balanced against the risk of developing dialysis disequilibrium syndrome.