Deep vein thrombosis, also called DVT, is a condition in which a blood clot (thrombus) develops in one of the body’s veins. DVT usually affects the legs, especially after long periods of travel or bed rest. It’s considered a serious condition because blood clots in your veins could potentially travel through your bloodstream and either block blood flow or become stuck in your lungs, which can lead to a potentially fatal condition called pulmonary embolism.