The NINDS NET-PD investigators are completing a study (called LS-1) to determine if a highly purified form of creatine can slow disease progression in people with Parkinson's disease. In this study, 1740 subjects have been assigned to take either 5 grams of creatine or a matching placebo (an inactive substance) twice per day for at least 5 years. Researchers will evaluate whether Parkinson's disease symptoms worsen at a slower rate in the subjects who are taking creatine compared to those taking placebo. The LS-1 study is no longer enrolling new subjects. At this time, it is in the process of following the enrolled subjects for the intended five year treatment period.