Context: Monotherapy with rasagiline mesylate may be useful in early Parkinson disease (PD). Objective: To evaluate the safety and efficacy of the selective monoamine oxidase type B inhibitor rasagiline. Design: Multicenter, 26-week, parallel-group, randomized, double-blind, placebo-controlled clinical trial. Setting: Academically based movement disorders clinics. Patients: Patients with early PD not requiring dopaminergic therapy (n=404). Intervention: Research participants were randomized to rasagiline mesylate at dosages of 1 mg or 2 mg per day or matching placebo. A 1-week escalation period was followed by a 25-week maintenance period. Main Outcome Measure: The primary prespecified measure of efficacy was the change in the total Unified Parkinson's Disease Rating Scal score between baseline and 26 weeks of treatment, comparing each active treatment group with the placebo group. Results: Monotherapy with rasagiline was effective in this 26-week study. The adjusted effect size for the total Unified Parkinson's Disease Rating Scale was -4.20 units comparing 1 mg of rasagiline and placebo (95% confidence interval, -5.66 to -2.73 units; P<.001) and -3.56 units comparing a 2-mg dosage and placebo (95% confidence interval, -5.04 to -2.08 units; P <.001). There were no meaningful differences in the frequency of adverse events or premature withdrawals among the treatment groups. Conclusions: Rasagiline is effective as monotherapy for patients with early PD. The 2 dosages in this trial were both effective relative to placebo. Further study is warranted to evaluate the longer-term effects of rasagiline in PD.