Treadmill Exercise and GM-CSF Study to Improving Functioning in Peripheral Artery Disease (PAD)

Inclusion Criteria: 1. Participants with an ankle brachial index (ABI) ≤ 0.90 will be eligible for participation. 2. Participants with an ABI \> 0.90 but ≤ 1.00 who experience a 20% drop in ankle pressure after the heel-rise exercise will be eligible. 3. Participants with an ABI \> 0.90 who have medical record evidence of prior lower extremity revascularization and experience a 20% drop in ankle pressure after the heel-rise exercise will be eligible for inclusion. 4. Participants with an ABI \> 0.90 who have medical record evidence of a non-invasive vascular laboratory test result consistent with PAD. Note that a screen-positive test from Lifeline Screening is not sufficient for inclusion in the study. Exclusion Criteria: The following exclusion criteria will be initially assessed by telephone: 1. Below or above-knee amputation. 2. Wheelchair confinement. 3. Use of a walking aid other than a cane (i.e. people using walkers). 4. Non-English speaking. 5. Significant hearing impairment. 6. Significant visual impairment. 7. Diagnosis of Parkinson's disease. 8. Inability to return to the medical center at the required visit frequency (three times per week). 9. \> Class II New York Heart Association heart failure or angina (symptoms at rest or with minimal exertion). 10. Any increase in angina pectoris symptoms during the previous 6 months or angina at rest. 11. Foot ulcer. (Participants with a foot ulcer will be excluded by telephone and/or during a baseline study visit). 12. Lower extremity revascularization in the last three months or major orthopedic surgery during the previous three months. 13. Myocardial infarction, stroke, or coronary artery bypass grafting during the previous 3 months. 14. Major medical illnesses including end stage renal disease requiring dialysis and chronic lung disease requiring oxygen, since these individuals may not be able to adhere to study requirements. Participants who only use oxygen at night may still qualify. 15. Potential participants who have received G-CSF, GM-CSF, or erythropoietin within the past year will be excluded because these interventions may influence study outcomes independently of the interventions. 16. Pre-menopausal women will be excluded because cyclic estrogen changes can influence progenitor cell levels. 17. Potential participants with diabetes and documented proliferative retinopathy will be excluded because GM-CSF may exacerbate this condition. 18. Potential participants with a history of myeloid malignancy will be excluded because GM-CSF may exacerbate these conditions. 19. Potential participants who have been treated for late stage cancer during the past three years, since GM-CSF may theoretically activate quiescent cancer cells. 20. Planned lower extremity revascularization within the next 6 months. 21. Current participation in another clinical trial. If a participant recently completed a clinical trial, at least three months must have passed before they can be considered for the PROPEL Trial. However, for a clinical trial of stem cell or gene therapy intervention, potential participants will be potentially eligible immediately after the final study visit of the stem cell or gene therapy clinical trial, so long as at least six months has passed since the participant received their final treatment in the stem cell or gene therapy intervention. 22. Walking for exercise at a level comparable to that targeted in our intervention. 23. Current participation in or completion of a cardiac rehabilitation program within the last six months. The following exclusion criteria will be assessed at the time of the study visit or later: 24. Severe aortic stenosis identified by physical exam at the study visit. 25. Critical limb ischemia identified by physical exam at the study visit. 26. Coronary ischemia during exercise, defined as ST segment depression \> 1 mm during the baseline exercise treadmill test, with or without associated chest discomfort, without a perfusion stress test demonstrating no reversible ischemia within the previous 3 months. 27. Left-bundle branch block or significant ST-T wave changes on the baseline ECG without a perfusion stress test demonstrating no reversible ischemia within the previous 3 months. 28. Stopping during the treadmill stress test for shortness of breath, chest pain, hip pain, knee pain, or another symptom that may not represent ischemic leg pain. 29. Stopping during the six-minute walk test for symptoms other than ischemic leg symptoms. 30. Foot ulcer identified at the study visit. 31. Mini-Mental Status Examination (MMSE) score \< 23 or disabling psychiatric disease. 32. Failure to complete a study run-in period. 33. Walking impairment due to a cause other than PAD. In addition to the exclusion criteria listed above, individuals thought to be poorly suited to the intervention (i.e. not a good fit) can be excluded at the discretion of the principal investigator.