Peripheral arterial disease is a common condition in older adults involving poor arterial circulation in the legs leading to leg pain and debility. The body's own circulating blood vessel stem cells may help to improve circulation. This study will test whether treatment with the drug granulocyte macrophage colony stimulating factor (GM-CSF) will improve symptoms and signs of peripheral arterial disease over placebo after four weeks of therapy. As well this study will examine whether improvements in blood vessel function can be observed. Finally, we will measure blood vessel function and stem cell levels in order to determine whether they can help to predict whether patients wither peripheral arterial disease will suffer further cardiovascular complications.