Thrombophlebitis (throm-boe-fluh-BY-tis) is an inflammatory process that causes a blood clot to form and block one or more veins, usually in your legs. The affected vein might be near the surface of your skin (superficial thrombophlebitis) or deep within a muscle (deep vein thrombosis, or DVT). Causes include trauma, surgery or prolonged inactivity. DVT increases your risk of serious health problems. It's usually treated with blood-thinning medications.