PD patients will then be randomized into two groups, placebo (n=8) and Leukine (n=8), and participation of the controls will end. Patients entered into the study will not have any noted co-morbid conditions including infection, inflammatory or cancerous diseases and will not be taking immunostimulatory or immunosuppressive medicines. Patients will continue to take medicines prescribed for the general treatment of PD, including, but not limited to, dopamine or dopamine agonists. There will be no history of prior surgeries linked to PD treatment. Patients will receive training for self-injection, and the treatment group will self-administer Leukine (6 µg/kg) by subcutaneous injection daily for eight weeks. The placebo group will self-administer saline as placebo. Every two weeks after the start of treatment and again 4 weeks after cessation of administration, all PD patients, receiving drug or placebo, will be evaluated by physical and clinical examinations, including UPDRS part III assessments. Every 2 weeks during Leukine treatment and again 4 weeks after cessation of drug, whole blood from PD patients will be obtained percutaneously using standard sterile techniques. At weeks 2, 4 and 8, results from CBC with WBC differential, FACS analysis for T cell markers, and Treg functional assays will be used to calculate immune response profiles and compared with baseline values. CBC with WBC differential, total T cell count, and blood chemistries will be analyzed every 2 weeks for evidence of potential toxicities, as well as 4 weeks after termination of Leukine treatment. Levels of anti-GM-CSF antibodies will be assessed for PD patients at enrollment and at 4 weeks intervals during drug treatment and 4 weeks after termination. Optional video recordings will be made at baseline, end of drug/placebo treatment and at follow-up.