Vegetarianism is one of the most struggling subjects in the field of Plutarchean studies. The paper aims at throwing light on the relationship between Plutarch's views on vegetarianism and some medical aspects he relies upon to support his attacks against eating meat. Plutarch mainly stresses his attention on human physiology, which is -according to his views- designed for a diet based on vegetables and fruits: that is why the first men on Earth used to be vegetarian; and, by the way, this is the reason why eating meat causes many different diseases, both physical and psychological. Moreover, by commenting some sections of the "De esu carnium", and comparing them with several passages from the "Moralia", the author shows that Plutarch used the same set of medical argumentations to defend vegetarianism throughout his vaste corpus.