The purpose of this study is to evaluate the efficacy, safety, and pharmacokinetics (PK) of prasinezumab compared with placebo in participants with early-stage Parkinson's disease (PD) on stable symptomatic monotherapy with levodopa.

Inclusion Criteria:

• Body weight within 40-110 kilograms (kg) (88-242 pounds [lbs]) and a body mass index within the range 18-34 kg/m2
• Diagnosis of idiopathic PD based on Movement Disorder Society (MDS) criteria
• Has received monotherapy treatment
• An MDS-UPDRS Part IV score of 0 at screening and prior to randomization
• Hoehn and Yahr (H&Y) Stage 1 or 2 off medication at screening and prior to randomization
• Agreement to adhere to the contraception requirements

Exclusion Criteria:

• Pregnant or breastfeeding, or intention of becoming pregnant during the study or within the time frame in which contraception is required
• Medical history indicating a parkinsonian syndrome other than idiopathic PD
• Diagnosis of a significant neurologic disease other than PD
• Chronic uncontrolled hypertension

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing