Successful leaders are at risk of developing exaggerated pride, contempt for others, and a diminished sense of reality. The ancient Greeks feared this syndrome and called it hubris. Although certain contemporaneous leaders show signs of hubris and pose a great danger, the hubris syndrome does not yet figure in our classification systems. The purpose of this paper is to examine several aspects of its validity, including clinical description, laboratory study, and exclusion of other disorders. Firstly, a substantial body of evidence indicates that the hubris syndrome may develop after a person has held substantial power for a considerable amount of time. Thus, the syndrome differs from a personality disorder with its characteristic onset in late adolescence or early adulthood. It is proposed, therefore, that the syndrome is a non-organic personality change after gaining substantial power or achieving overwhelming success, characterized by the emergence or marked increase of pathological personality traits within the domains of dissociality and disinhibition. Within the domain of dissociality, grandiosity is an obligatory trait. Secondly, with reference to laboratory study, recent evidence suggests that machine learning algorithms have the ability to differentiate hubristic from non-hubristic speech patterns. Thirdly, the exclusion of other disorders is difficult, because individuals with the hubris syndrome do not collaborate in any investigation. Some suggestions are made to overcome this problem. In conclusion, there is sufficient reason to further examine the validity of the hubris syndrome and to consider it for inclusion in our classification systems.