Peripheral vascular disease refers to a narrowing of the blood vessels in the extremities, typically the legs and feet, which leads to lack of adequate blood supply. Patients with moderate peripheral vascular disease develop leg and foot pain during activity, a condition called claudication. When the disease is more severe, patients experience pain even without exercise, a condition known as pain at rest. When the disease is most severe, tissues that do not get enough blood can die, and the patient develops sores on the leg or foot (ischemic ulcers or gangrene). A new experimental therapy for peripheral vascular disease involves injecting patients with a gene that allows cells to produce vascular endothelial growth factor, a substance that helps tiny new blood vessels to grow. Some researchers have noticed that patients who get this kind of gene therapy develop the complication of swelling in their legs.