Deep vein thrombosis is a condition in which a blood clot (thrombus) forms in the deep veins of the legs, pelvis, or arms. These veins are located near the bones and are surrounded by muscle.

A thrombus may form in the deep veins as a result of a blood-clotting abnormality, an injury, or prolonged inactivity (such as a long airplane ride or bed rest).

A deep vein thrombus can break loose and travel through the bloodstream to the lung. This is called pulmonary embolism and can be dangerous.

A person with deep vein thrombosis may or may not have symptoms. If symptoms are present, they often include tenderness, pain, or swelling.

Treatment usually includes the use of blood-thinning medicines (anticoagulants) that prevent new clots from forming or prevent existing clots from getting larger.