Objectives: The primary objectives are: (1) to quantify the absolute risk of major hemorrhagic complications associated with anticoagulation for AF among octogenarians; (2) to determine if risk and severity of major hemorrhagic complications is increased in octogenarians compared with younger patients maintained on warfarin for AF after adjustment for potential confounders; (3) to explore predictors of major hemorrhagic complications among octogenarians maintained on warfarin for AF; and (4) to determine the effectiveness of utilizing the VA Intranet to collect and manage patient data in the conduct of longitudinal studies.