BACKGROUND: Although the presence of amyloid deposits is associated with a more severe cognitive status in patients with stroke at baseline, its influence on the subsequent cognitive outcome has not been extensively assessed. The primary objective of the present study of the IDEA3 (Imagerie des dépôts amyloïdes cérébraux par florbetapir AV-45 et diagnostic des déficits cognitifs et démence post Accident Vasculaire Cérébral) cohort was to determine the influence of amyloid positron emission tomography (PET) status on the 5-year cognitive outcome. METHODS: This longitudinal study performed in Amiens University Hospital (inclusions: October 2014 to October 2019; last visits: October 2018 to February 2023) has included 91 patients with stroke (ischemic stroke, 89%; hemorrhagic stroke, 11%) with florbetapir PET data at baseline (positive, n=14). Patients underwent annually comprehensive clinical and cognitive assessments for 5 years after the PET scan. The primary outcome was incident dementia; secondary outcomes were incident cognitive impairment, total prevalence of cognitive impairment, and modified Rankin Scale score. RESULTS: A survival analysis (mean poststroke follow-up, 80.4±27 months) showed that the incidence of incident dementia was higher in the PET-positive patients (odds ratio, 9.6 [95% CI, 2.5–36.9]; P =0.001), as was the incidence of incident cognitive impairment (odds ratio, 10 [95% CI, 1.9–52.3]; P =0.003). A Cox regression analysis showed that the association between amyloid status and the incidences of dementia ( P =0.001) and cognitive impairment ( P =0.007) was still significant after adjustment for age, education, prestroke modified Rankin Scale score and cognitive impairment, stroke type, and the PET status×stroke type interaction. Considering the overall prevalence at the last follow-up in the whole study population (n=91 patients), PET positivity was associated with an elevated risk of dementia (odds ratio, 6 [95% CI, 1.76–20.5]; P =0.002) and poststroke cognitive impairment (odds ratio, 6.25 [95% CI, 1.77–22]; P =0.002). The final modified Rankin Scale score did not differ ( P =0.3) according to PET status. CONCLUSIONS: Our results demonstrate the major impact of amyloid deposition on the stroke outcome and emphasized the need for comprehensive etiologic workup in patients with poststroke cognitive impairment. REGISTRATION: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT02813434.