Deep vein thrombosis is the term for a blood clot that develops in one of the deep veins of the body. Though clots usually form in one of the deep veins of the leg, they have also been known to appear in the arm, brain, or eye. Deep vein thrombosis develops when blood flow within a vein is reduced and/or the patient has a family history of deep vein thrombosis. The clot produces pain, redness, swelling, and warmth in the affected area. Though deep vein thrombosis is fairly unusual (it only affects one to two people per 1,000), its appearance should not be taken lightly. Clots that are left untreated can become dislodged and move to other parts of the body where their presence is life threatening.