Deep venous thrombosis (DVT) is essentially a clot in one of the large, deep leg veins returning blood from the foot and leg to the heart. This condition blocks the return of blood through that vein, and therefore the fluid accumulates below the point of the clot, causing swelling and inflammation. The danger this clot poses is severe, as it can potentially break off the wall of the vein and move up the body. This path follows the veins through the 1st half of the heart, to eventually clog the arteries in the lungs where the blood receives oxygen. When this happens, the body cannot survive as the blood becomes deprived of vital oxygen, and a quick death occurs. This condition is called a pulmonary embolus, and symptoms include difficulty breathing, rapid pulse, sweating, and sometimes chest pain with blood produced during coughing. Immediate emergency treatment is needed to prevent death.

DVTs develop due to a combination of three elements. The first of these three elements is inflammation in the wall of the vein. Any actual structural or chemical abnormality in the vein wall tissue itself allows for the material of a clot to stick to the wall, and this attracts more clot to attach to the initial clot. The second element is decreased flow of blood in the vein. This can be due to immobilization during bed recovery from surgery or the use of a cast for an injury. It can also occur naturally when there are varicosities of the veins and chronic leg swelling that indicates poor vein functioning. The final element contributing to DVT is the presence of blood that can clot easier. This occurs after injury, after giving birth, with some types of cancer, and during birth control pill use.

Symptoms of a DVT can vary, but almost always some form of the lower leg or calf aching, tightness, or outright pain. Swelling is also usually present, and redness and warmth of the leg can be present in many cases, but not all. Pain when compressing the calf or when flexing the ankle upward can possibly indicate the presence of a DVT, although these symptoms are nonspecific as a sore or bruised calf muscle may also be present. Other explanations for some of these symptoms could include infection, inflammation of superficial veins that pose no risk to the deep veins, bruising, or a clot in the artery. Proper and timely diagnosis of a DVT is very important, since the symptoms can be nonspecific and the risk dire if that clot breaks off and heads to the lungs. Usually, a trip to an emergency room or hospital vascular centre is necessary, where the proper equipment is available to make a proper diagnosis. Many blood clots are diagnosed with the use of an ultrasound instrument, although some clots require the use of dye and x-ray in order to see them (venography).

Treatment of DVTs involves rest and medicine. The leg needs to be rested and elevated to reduce the swelling. Compression bandages can be used as well. Medication to thin the blood, usually, an injected drug called heparin is used to decrease the potential of clots to continue forming and stick to the wall of the vein. Eventually, an oral medication called warfarin is used, possible for numerous months, to continue the thinning. It takes a little over a week for vein clots to become firmly attached to the vein wall, and eliminate the risk of the clot breaking off and heading to the lungs. However, in some people, the risk of further clotting persists, and blood-thinning medication may be a need for quite some time to reduce this risk. In severe cases, a mesh filter maybe needs to be inserted in the primary vein before the heart to 'catch' clots before they can travel to the lungs.