A blood clot (a clump of blood) that forms in a vein and causes inflammation is called thrombophlebitis. Symptoms include pain, tenderness, warmth, and redness along the length of a vein.

A clot may form either in a vein just under the surface of the skin (superficial thrombophlebitis) or in a vein deep in the leg (deep vein thrombosis, or DVT). Clots in a vein just under the skin rarely cause serious problems because they do not travel through the bloodstream. Once this kind of blood clot has been diagnosed by a health professional, the person can often care for the symptoms safely at home.

Thrombophlebitis can occur in any vein, but it is most common in the leg veins. Clots in a deep leg vein (deep vein thrombosis) are serious because a clot can break loose and travel through the bloodstream to the lung (pulmonary embolism). Symptoms of deep vein thrombosis include swelling, pain, and possibly redness in one leg or a noticeable new difference in the size of one leg. Another symptom of deep vein thrombosis is pain when walking or when the foot is flexed upward.