This study is designed as a prospective cohort study to test the strategy of combining two biomarkers of parkinsonism, olfaction and brain imaging with a radioactively labeled drug, [123I]β-CIT , in a population of first-degree relatives of PD patients as a tool to establish an 'at risk' Parkinson disease cohort without motor symptoms of PD. First-degree relatives of PD will be recruited through PD research sites and national foundations to participate in this study. In addition, first degree relatives of PD patients will be recruited directly through advertising.