For more than two thousand years, almost all doctors in the West used bloodletting to treat a great variety of diseases and conditions. In an attempt to find out why they acted thus, Dr Brain has translated the three works on bloodletting by the second-century physician Galen, which provide by far the most comprehensive account of the practice in antiquity. This is the first published version of these works in a modern language. After a brief summary of Galen's medical system, the author goes on to consider the origins of Galen's ideas and methods, with particular reference to the Hippocratic writings, and the question why Galen, in common with most of the ancient physicians, believed in the efficacy of the comedy. The effects of bloodletting are considered in terms of modem physiology and medicine, and the possibility is explored that it may indeed have been beneficial in the conditions prevailing in Galen's time.