BACKGROUND: In the AUGUSTUS trial (An Open-Label, 2×2 Factorial, Randomized Controlled, Clinical Trial to Evaluate the Safety of Apixaban Versus Vitamin K Antagonist and Aspirin Versus Aspirin Placebo in Patients With Atrial Fibrillation and Acute Coronary Syndrome or Percutaneous Coronary Intervention), apixaban resulted in less bleeding and fewer hospitalizations than vitamin K antagonists, and aspirin caused more bleeding than placebo in patients with atrial fibrillation and acute coronary syndrome or percutaneous coronary intervention treated with a P2Y12 inhibitor. We evaluated the risk-benefit balance of antithrombotic therapy according to kidney function. METHODS: In 4456 patients, the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) formula was used to calculate baseline estimated glomerular filtration rate (eGFR). The effect of apixaban versus vitamin K antagonists and aspirin versus placebo was assessed across kidney function categories by using Cox models. The primary outcome was International Society on Thrombosis and Haemostasis major or clinically relevant nonmajor bleeding. Secondary outcomes included death or hospitalization and ischemic events (death, stroke, myocardial infarction, stent thrombosis [definite or probable], or urgent revascularization). Creatinine clearance <30 mL/min was an exclusion criterion in the AUGUSTUS trial. RESULTS: Overall, 30%, 52%, and 19% had an eGFR of >80, >50 to 80, and 30 to 50 mL·min-1·1.73 m-2, respectively. At the 6-month follow-up, a total of 543 primary outcomes of bleeding, 1125 death or hospitalizations, and 282 ischemic events occurred. Compared with vitamin K antagonists, patients assigned apixaban had lower rates for all 3 outcomes across most eGFR categories without significant interaction. The absolute risk reduction with apixaban was most pronounced in those with an eGFR of 30 to 50 mL·min-1·1.73 m-2 for bleeding events with rates of 13.1% versus 21.3% (hazard ratio, 0.59; 95% CI, 0.41-0.84). Patients assigned aspirin had a higher risk of bleeding in all eGFR categories with an even greater increase among those with eGFR >80 mL·min-1·1.73 m-2: 16.6% versus 5.6% (hazard ratio, 3.22; 95% CI, 2.19-4.74; P for interaction=0.007). The risk of death or hospitalization and ischemic events were comparable to aspirin and placebo across eGFR categories with hazard ratios ranging from 0.97 (95% CI, 0.76-1.23) to 1.28 (95% CI, 1.02-1.59) and from 0.75 (95% CI, 0.48-1.17) to 1.34 (95% CI, 0.81-2.22), respectively. CONCLUSIONS: The safety and efficacy of apixaban was consistent irrespective of kidney function, compared with warfarin, and in accordance with the overall trial results. The risk of bleeding with aspirin was consistently higher across all kidney function categories. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT02415400.