Idiopathic hyperammonemic encephalopathy is a rare complication of chemotherapy, which has previously mainly been associated with L-asparaginase, cytarabine and 5-FU. We present a case following treatment with gemcitabine–cisplatin in a patient with cholangiocarcinoma. The etiology of chemotherapy-induced idiopathic hyperammonemic encephalopathy remains unclear and existing theories differ per chemotherapeutic agent. Physicians treating patients with gemcitabine–cisplatin should be aware of the possibility of this complication, especially because it is treatable when recognized early.