This paper centers on the relationship between renaissance medicine and antitrinitarianism forming an important part of the radical reformation in 16th century. Antitrinitarians denied the theological concept of the triune Christian God. Modern antitriniarianism was first formulated by Michael Servetus, a Spanish physician. The doctrine soon won disciples in northern Italy (Padua, Pisa, Rome) and in some countries of eastern central Europe (Poland, Moravia, Transylvania). Physicians who believed in or sympathized with antitrinitarianism revolutionized the theory of blood flow. They developed new concepts leading to Harvey's theory of the circulation of the blood. Parts of Servet's theory looked like Paracelsus's mystical idea of the eternal flesh of the body of Christ (Latin limbus aeternus), but later the medicine and natural philosophy of antitrinitarian physicians tended to be empiristic, rational and sometimes materialistic. They contributed heavily to the diffusion and physiological foundation of antitrinitarian concepts like unitarianism, tolerance and irenism.