Rationale About 20% of stroke patients develop dementia within a few months after their event, but the determinants and mechanisms of poststroke dementia are insufficiently understood. Aims To identify and characterize the determinants of cognitive impairment poststroke. Design Observational prospective study in patients with acute stroke and no prior dementia. Six hundred subjects will be characterized by detailed interview, standardized clinical examinations, biometric measures (intima-media thickness, waist-hip ratio, and ankle-brachial index), multimodal imaging (magnetic resonance imaging, fluorodeoxyglucose-positron emission tomography (FDG-PET), amyloid-positron emission tomography (amyloid-PET), and retinal imaging), analysis of biomarkers derived from blood and cerebrospinal fluid, and detailed cognitive testing at repeat time points. Patients will be followed for five-years with a total of five personal visits and three telephone interviews. Study Outcomes Primary end-point is the occurrence of post-stroke dementia. Secondary end-points include poststroke cognitive impairment–no dementia, stroke recurrence, and death. Predictive factors for poststroke dementia will be identified by multiple Cox proportional-hazards model. Results Baseline characteristics of the first 71 patients (study inclusion between May 2011 and August 2012) are as follows: median age, 70 years (interquartile range, 65–75); female gender, 25 (35%); median National Institutes of Health Stroke Scale at admission, 2 ( 1 – 4 ); and etiological stroke subtypes according to TOAST classification, 15% large artery disease, 18% small vessel disease, 35% cardioembolic, and 32% undetermined or multiple competing etiologies. Discussion This study will provide insights into the mechanisms of poststroke dementia and hold the potential to identify novel diagnostic markers and targets for preventive therapies. The study is registered at http://www.clinicaltrials.gov (NCT01334749) and will be extended as a multicenter study starting 2013.