In this paper, the original Greek language texts of the Byzantine medical literature about lycanthropy are reviewed. The transformation of a human being into a wolf and the adoption of animal-like behaviour, which were already known from mythology and had been presented in the scientific works of ancient Greek and Roman physicians, were examined by six Byzantine physicians and explained as a type of melancholic depression or mania. In spite of the influence of Byzantine medicine, its rationality in the interpretation of lycanthropy was forgotten in medieval and Renaissance times when it was replaced by explanations based on demonic possession and witchcraft. More recently psychiatry has treated the phenomenon as a subject of medical inquiry and has again explained the condition in terms of mental disorder.