Valproate is commonly used in various psychiatric and neurologic settings and is usually well tolerated. We present the case of a 26-year-old man who developed encephalopathy after valproate treatment. Ten days after initiating valproate therapy, he presented with a sudden decreased level of consciousness, disorientation, seizures, and eventually coma. Notably, the serum ammonia level was elevated to 392 μmol/l with no hepatic dysfunction on laboratory tests. He did not have a urea cycle-associated enzyme deficiency on tandem mass spectrometry. However, the free carnitine level was low (5 μmol/l), and this was suspected of being caused by the valproate treatment. Due to the possibility of valproate-induced hyperammonemic encephalopathy, the valproate was discontinued, and the patient was treated with L-carnitine 50 mg/kg per day. He recovered completely within 3 days of L-carnitine treatment, and the free carnitine level was within normal limits (53.5 μmol/l) 5 months thereafter