Venous thrombosis is the formation of a blood clot in a vein. It usually occurs in a vein in the lower limbs, in the leg or hip, rarely in the upper limbs. Venous thrombosis can be superficial when it touches a vein close to the surface of the skin or deep when it is located inside a muscle. Most often, venous thrombosis is accompanied by phlebitis, an inflammatory reaction of the vein when the clot grows and adheres to the wall. In practice, a deep vein thrombosis is usually called "phlebitis". The course of deep vein thrombosis can be serious, even fatal if the clot breaks off, circulates in the blood and clogs an artery in the lung causing a pulmonary embolism.