Two adult patients with focal epilepsy who tolerated valproate (VPA) well in different combinations of anticonvulsants developed hyperammonemic encephalopathy when treated with VPA in combination with topiramate (TPM). Topiramate may contribute to the increased ammonia level by its inhibition of carbonic anhydrase and cerebral glutamine synthetase, thereby facilitating VPA/TPM hyperammonemic encephalopathy. Recovery occurred after withdrawal of VPA or TPM.