The study is designed to further examine the extent to which GM1 ganglioside can improve symptoms, delay disease progression, and, perhaps, partially restore damaged brain cells in PD patients. GM1 ganglioside is a chemical that is normally found in the brain and is a normal part of the outer covering or membrane of nerve cells. This study will compare the effectiveness of GM1 to standard PD treatment. In addition to studying clinical measures of motor and cognitive functioning, the investigators will use PET (positron emission tomography) scanning to image the brain and the dopamine nerve endings in a subgroup of patients. Patients with mild to moderate idiopathic PD will be divided into 2 groups. One group will receive GM1 for 24 weeks and the other will receive placebo. At the end of this 24 week period, all patients will enter into a 96 week treatment period in which all patients will receive GM1.