Lycanthropy, an unusual psychiatric syndrome involving the delusion of being an animal, usually occurs as a transient symptom of severe psychosis. A review of the historical and modern medical literature, as well as of contemporary anthropological reports, suggests multiple etiologies for lycanthropy, including seizure disorders and use of psychotomimetic drugs. A clinical illustration is presented in which the delusion of being an animal in human form has persisted for over 15 years and has been refractory to treatment. The authors speculate that disturbances of self-identity may combine with neurological abnormalities to produce some cases of this syndrome.