Deep venous thrombosis (DVT) affects mainly the veins in the lower leg and the thigh. It involves the formation of a clot (thrombus) in the larger veins of the area. This thrombus may interfere with circulation of the area, and it may break off and travel through the blood stream (embolize). The embolus thus created can lodge in the brain, lungs, heart, or other area, causing severe damage to that organ.

Risks include prolonged sitting, bedrest or immobilization (such as on long plane or car trips), recent surgery or trauma (especially hip, knee or gynecological surgery), fractures, childbirth within the last 6 months and the use of medications such as estrogen and birth control pills. Risks also include a history of polycythemia vera, malignant tumor, and inherited or acquired hypercoagulability (changes in the levels of blood clotting factors making the blood more likely to clot).

Deep venous thrombosis is more commonly seen in adults over age 60 but can occur in any age group.