Blood clots can be lifesavers when they form outside the bloodstream to stop bleeding from an injury. But they can wreak havoc when they form inside the bloodstream. A blood clot in a coronary artery can cause a heart attack. One in the brain can cause a stroke. Blood clots that form in a leg vein cause a problem known as venous thromboembolism, or VTE. If the clot stays in the leg, it can cause swelling or pain. If it breaks away and travels to the lungs, it can cause a potentially deadly pulmonary embolism. In about half of people who develop a VTE, doctors can identify what caused it. In the other half, VTE is something of a mystery. These are called “unprovoked” VTEs. Such unprovoked VTEs often spark a search for hidden cancer. But a study published in The New England Journal of Medicine suggests that these searches are usually fruitless — and costly.