The adult's mastery and usage of several languages entails phenomena of medical-particularly neurologic and psychiatric-import. This review depicts psychopathologic features of polyglottism, including potential eruption of panic in early stages of an adult's acquisition of a new tongue (incipient bilingualism) and useless construction of complex, artificial but "true" languages by certain bright psychotics and psychopaths. The often sparing effects of polyglottism on malcommunication of dysphasics and schizophrenics are described. Finally, benefits and detriments of polyglottism in psychotherapy are noted. Despite much basic research in polyglottism done in linguistics and psychology with healthy subjects the medical-particularly the neurologic and psychiatric-literature, is to a great extent limited to descriptions of isolated cases of polyglot aphasics, psychotics, and psychiatric patients. More systematic clinical research is needed.