Blood clots can form in arteries or veins. When inflammation due to a blood clot occurs in a vein, it is called thrombophlebitis. Thrombophlebitis usually occurs in the veins of the legs. Rarely, it can occur in the veins of the arms or neck. Superficial (near the surface) thrombophlebitis occurs in the veins that are visible just under the skin. The area of inflammation is usually reddened, tender, and warm to the touch and can be painful. The leg or arm may swell and fever may occur. Deep venous thrombosis (DVT) is more dangerous than superficial thrombophlebitis. Deep venous thrombosis often cannot be seen or felt by the individual. Swelling of the leg or arm or fever may alert a person to the presence of a DVT, especially if risk factors for DVT exist. Pulmonary embolism, a condition that can be fatal, results from a DVT that becomes loose in the venous system and travels to the lungs. There, the blood clot blocks proper blood flow to the lungs and decreases oxygen levels in the body. The October 8, 2008, issue of JAMA includes an article about diagnostic testing for DVT. This Patient Page is based on one previously published in the July 26, 2006, issue of JAMA.