Parkinson's disease (PD) is a progressive and disabling neurological disorder involving the nigrostriatum and for which no cure is known. Evidence suggests that inflammation contributes, significantly, to the cause and/or progression of the disease. Studies in postmortem human brain, animal models, and human peripheral blood support the idea that the frequency and function of particular circulating T cell subsets are abnormal and worsen as disease progresses. This abnormality is linked specifically to increases in numbers of T effector neurodestructive cells (Teff) and to dysfunction of regulatory T cells (Treg) that control Teff. This imbalance tips the homeostatic balance to a pro-inflammatory profile with limited control. Nonetheless, whether such T cell deficits are a cause of PD or a reaction to it remains unknown. If these deficits are causal or exacerbative, then correcting the deficit could have significant positive effects on disease progression and could ameliorate nigrostriatal degeneration or its sequelae. Granulocyte macrophage colony stimulating factor (GM-CSF) is a potential immunomodulatory therapeutic for PD to increase Treg numbers or function and reduce or transform proinflammatory Teff responses, leading to neuroprotection of the nigrostriatum and improved clinical outcomes for disease. Recombinant human GM-CSF (sargramostim) is available as Leukine®. The purpose of this study is to determine if Leukine (sargramostim) can be safely administered to Parkinson's disease patients for an extended period of time (56 days).