Deep vein thrombosis (DVT) occurs when a blood clot (thrombus) forms in one or more of the deep veins in your body, usually in your legs. Deep vein thrombosis can cause leg pain or swelling but also can occur with no symptoms.

You can get DVT if you have certain medical conditions that affect how your blood clots. A blood clot in your legs can also happen if you don't move for a long time, such as after you have surgery or an accident, when you're traveling a long distance, or when you're on bed rest.

Deep vein thrombosis can be very serious because blood clots in your veins can break loose, travel through your bloodstream and get stuck in your lungs, blocking blood flow (pulmonary embolism). However, pulmonary embolism can occur with no evidence of DVT.

When DVT and pulmonary embolism occur together, it's called venous thromboembolism (VTE).