Background Patients with atrial fibrillation (AF) and chronic kidney disease represent a high-risk group for thromboembolism and bleeding. Aims To explore the relationship between kidney function changes and outcomes of stroke/systemic embolism (SE), major bleeding and all-cause death in anticoagulated AF patients participating in the BOREALIS trial comparing efficacy and safety of once-weekly s.c. idrabiotaparinux to that of warfarin. Methods Changes in kidney function by estimated glomerular filtration rate (eGFR) were calculated using the Chronic Kidney Disease Epidemiology Collaboration equation in 2765 AF patients. Trial adjudicated outcomes were determined. Results After a mean follow-up of 394 days, in 94.4% of the included patients kidney function changed ranging from -30 mL/min to 30 mL/min. The incidence of stroke/SE and major bleeding was similar between patients with deteriorated (reduction in eGFR from baseline over follow-up) and preserved kidney function change (increase or no change in eGFR from baseline over follow-up) [stroke/SE: incidence rate (IR): 1.33%/year vs 1.80%/year; hazard ratio (HR) 0.74, 95% confidence interval (CI) 0.41-1.32, P = .30; major bleeding: IR 1.63%/year vs 1.49%/year, HR 1.10, 95% CI 0.61-1.97, P = .76]. On Cox regression analysis, patients with deteriorated kidney function were at higher risk for all-cause death, compared to patients with preserved kidney function (HR: 1.64, 95% CI: 1.02-2.63, P = .04). Conclusion In the BOREALIS trial, the risk of adjudicated stroke/SE, major bleedings, and all-cause death was not related to mild-moderate follow-up changes in kidney function (+/- 30 mL/min). The risk of all-cause death was significantly increased in AF patients with abruptly deteriorating kidney function.