Background 200 million patients undergo noncardiac surgery every year. Overt stroke after noncardiac surgery is not common, but has a substantial impact on duration and quality of life. Covert stroke in the nonsurgical setting is much more common than overt stroke, and associated with an increased risk of cognitive decline and dementia. Little is known about covert stroke after noncardiac, noncarotid artery surgery. Methods We undertook a prospective cohort pilot study to inform the incidence of covert stroke after noncardiac, noncarotid artery surgery, and to determine the feasibility of a full prospective cohort study to characterize the epidemiology of perioperative covert stroke. Patients underwent a brain MRI study between postoperative days 3-10, and were followed up at 30 days after surgery. Results of the pilot study We enrolled a total of 100 patients from 6 centres in 4 countries, demonstrating excellent recruitment and no loss to follow-up at 30 days after surgery. The incidence of perioperative covert stroke was 10.0% (10/100 patients, 95% confidence interval 5.5% to 17.4%). Full study protocol We describe a proposal for a prospective cohort study of 1,500 patients. An MRI study of the brain will be performed between postoperative days 2 and 9. The primary outcome is cognitive function, measured 1 year after surgery using the Montreal Cognitive Assessment tool. We will perform multivariable logistic regression analysis where the dependent variable is the change in cognitive function 1 year after surgery, and the independent variables are incidence of perioperative covert stroke and other risk factors for cognitive decline. Conclusions This international multicentre pilot study suggests that 1 in 10 patients ≥65 years of age experiences a perioperative covert stroke. The proposed protocol describes a larger study which will determine the impact of perioperative covert stroke on patient-important outcomes. / Thesis / Master of Science (MSc) / This thesis describes a program of research to investigate silent stroke after surgery.