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Objective: In patients with acute mental status change with elevated serum ammonia levels, when there is conco-mitant use of valproate, carbamazepine or any other susceptible agent, hyperammonemic encephalopathy is a possible phenomenon. Our aim was to describe this underreported phenomenon in detail and find possible risk factors associated with valproate induced hyperammonemic encephalopathy. Methods: We analyzed retrospective records of two years for female inpatients in the psychiatric ward for women in our hospital. Spearmans rho correlation analysis was used for assessing the relation between serum ammonia and valproate levels. During the two years period, 961 patients were treated in the inpatient clinic. Because of a suspicion of hyperammonemic encephalopathy, serum ammonia levels of 37 patients were analyzed, revealing a total of 34 patients with high ammonia levels, warranting a diagnosis of hyperammonemic encephalopathy. Results: In 28 of these patients hyperammonemic encephalopathy was associated with valproate use. The time interval between initiation of valproate treatment and hyperammonemia was 11±7 days. For these patients, initiation dose of valproate was 883.93±249.83 mg/day (median: 1000 mg/day), daily valproate dose was 914.90±202.87 (median: 928.25) mg and during encephalopathy; valproate blood level was 82.36±25.80 ng/mL (median: 87.50 ng/mL). Serum ammonia level was positively correlated with the initiation dose of valproate ((28)=0.472, p=0.011) and valproate blood levels during hyperammonemia ((28)=0.522, p=0.004). Polypharmacy and long-term intramuscular treatment were also prevalent in patients with hyperammonemic encephalopathy. Discussion: Early suspicion of hyperammonemic encephalopathy in patients taking valproate may improve the outcome of this under recognized problem. [Anadolu Psikiyatri Derg 2019; 20(6.000): 589-596]