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 (on the surface) thrombophlebitis occurs in the visible veins just under the skin. The area of inflammation is usually reddened, tender, and warm to the touch and can be painful. The extremity may swell and fever may occur. Deep vein thrombosis (DVT) is more dangerous than superficial thrombophlebitis. Deep vein thrombosis often cannot be seen or felt by the individual. Swelling of the leg 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 veins and travels to the lungs. There, it blocks proper blood flow to the lungs and decreases oxygen levels in the body. The April 6, 2011, issue of JAMA includes an article about thromboembolism (clots being carried by the bloodstream). This Patient Page is based on one previously published in the July 26, 2006, issue of JAMA.