Bloodletting (or blood-letting) is the withdrawal of blood from a patient to cure or prevent illness and disease. Bloodletting was based on an ancient system of medicine in which blood and other bodily fluids were regarded as "humors" that had to remain in proper balance to maintain health. It is claimed to have been the most common medical practice performed by surgeons from antiquity until the late 19th century, a span of almost 2,000 years. In Europe the practice continued to be relatively common until the end of the 18th century. The practice has now been abandoned by modern style medicine for all except a few very specific conditions. It is conceivable that historically, in the absence of other treatments for hypertension, bloodletting could sometimes have had a beneficial effect in temporarily reducing blood pressure by reducing blood volume. However, since hypertension is very often asymptomatic and thus undiagnosable without modern methods, this effect was unintentional. In the overwhelming majority of cases, the historical use of bloodletting was harmful to patients. Today, the term phlebotomy refers to the drawing of blood for laboratory analysis or blood transfusion. Therapeutic phlebotomy refers to the drawing of a unit of blood in specific cases like hemochromatosis, polycythemia vera, porphyria cutanea tarda, etc., to reduce the number of red blood cells. The traditional medical practice of bloodletting is today considered to be a pseudoscience.

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