After a short historical review of the contemporary medical literature, the authors analyze a new and original observation of lycanthropy. He is a 28 years old man, imprisoned for deadly violence, who has been showing, for many years, the belief of being transformed into a werewolf during depersonalization episodes when he presents a lycanthropic behaviour. Our observation is closer to hysteria and mythomania on an antisocial personality, although it seems difficult to place the mental pathology of this alcoholic recidivist delinquent into a nosographical frame.