First-degree relatives that agree to participate (n=3,000) will be asked to complete a 40-item olfactory identification test provided by mail. 300 subjects (225 with decreased odor identification and 75 with normal olfaction) will be invited to undergo DAT imaging at the Institute for Neurodegenerative Disorders in New Haven, CT. There will also be additional clinical follow-up at participant's clinical (local) site. The primary outcome measure for the study will be the mean striatal uptake of [123I]B-CIT in first-degree relatives with a loss of odor identification, which will be compared to an established healthy control database (age 40-70; n=50). 300 relatives will be followed longitudinally with clinical evaluations and a second imaging study completed after two years. Comparing the first and second scans in this subset of subjects will allow us to evaluate the rate of progressive loss in dopamine transporter density during this pre-symptomatic period.