In pathological mixing multilingual patients mix two or more languages within a single utterance, whereas in pathological switching patients alternate utterances in one language to utterances in another, even when the interlocutor cannot understand one of the two languages. Numerous studies have established that pathological mixing is mainly due to lesions in the parietotemporal structures of the left hemisphere, whereas the nervous structures responsible for switching between languages have not yet been clearly described.5 6 The study of the bilingual patient reported here has allowed us to establish—for the first time—the role of anterior brain structures in the switching mechanism in multilingual subjects.