Evidence is accumulating that postoperative delirium is a marker of brain vulnerability. Cognitive prognosis following delirium remains unclear. This project aims to test the hypothesis that participants who developed postoperative delirium will have worse cognition eight years later compared to those who did not develop postoperative delirium and the effect of delirium will be independent of preoperative cognition and biomarkers. This project will follow up participants previously recruited between 2012 and 2014 to an observational cohort study (n=315) investigating postoperative delirium following elective orthopaedic surgery which sampled DNA, plasma and cerebrospinal fluid. Participants will be invited to a one-off follow-up study visit where updated medical history, repeat cognitive assessments, collateral history taking and blood sampling for neurofilament light analysis will be completed. It is anticipated that the results of this study will inform individualised prognosis following postoperative delirium. This in turn will strengthen the case for national and international guidelines regarding postoperative patient counselling, follow-up care planning and intervention to improve brain health and potentially modify Alzheimer’s disease processes after postoperative delirium. If delirium is shown to independently predict subsequent cognitive decline, even when baseline vulnerability is comprehensively corrected for, this will have further implications for the prevention of dementia on a global scale. Given available effective multicomponent interventions to reduce risk, it could also provide a modifiable risk factor for dementia.