Perhaps the most important unmet need in Parkinson's disease (PD) is the ability to slow or prevent progression of the neurodegeneration that underlies the motor and nonmotor features of this disorder. Pramipexole, a dopamine agonist used for the symptomatic treatment of PD, has demonstrated neuroprotective properties in laboratory studies. The PRamipexole On Underlying Disease (PROUD) study is a randomized, double-blind clinical trial evaluating the ability of pramipexole to modify disease progression using a delayed-start design. PD patients (n = 535) with mean age 62.5 years, mean duration since diagnosis of 4.4 months, and mean total Unified Parkinson's disease Rating Scale (UPDRS) score of 24.5 were recruited. In Phase I, patients were randomly assigned to be titrated to 1.5 mg pramipexole or placebo and maintained on study drug for 6-9 months. In Phase II, all patients were titrated to 1.5 mg pramipexole and maintained on study drug until the end of the study at 15 months. No rescue medication was allowed in the protocol. The primary endpoint is the change in total UPDRS score (parts I-III) from baseline to 15 months. A range of secondary endpoints separately assess UPDRS subscales, quality of life, depression, and impulse control disorders. A sub-study examined dopamine transporter uptake scans at baseline and 15 months. The results of PROUD will provide insight into the potential for early versus delayed treatment with pramipexole to modify motor outcome at 15 months in recently diagnosed PD patients.