AIMS: Atrial fibrillation increases the risks of stroke, heart failure, and death, and anticoagulation therapy increases the risk of gastrointestinal haemorrhage. However, the relative event rates for these outcomes are not well described. We sought to define the risks of major clinical events in older adults after a new diagnosis of atrial fibrillation. METHODS AND RESULTS: We undertook a population-based, retrospective cohort study of a nationally representative sample of fee-for-service Medicare beneficiaries 65 years or older with incident atrial fibrillation diagnosed between 1999 and 2007. The main outcome measures were mortality and hospitalization or emergency department care for heart failure, myocardial infarction, stroke, or gastrointestinal haemorrhage. Among 186 461 patients with atrial fibrillation and no recent hospitalizations for heart failure, myocardial infarction, stroke, or gastrointestinal haemorrhage, mortality was the most frequent of these major clinical events (19.5% at 1 year; 48.8% at 5 years). By 5 years, 13.7% of patients were hospitalized for heart failure, 7.1% developed new-onset stroke, and 5.7% had gastrointestinal haemorrhage. Myocardial infarction was less frequent (3.9% at 5 years). Rates of mortality, heart failure, myocardial infarction, stroke, and gastrointestinal bleeding increased with older age and higher CHADS2 scores. Among 44 479 patients with previous events, the 5-year risk of death was greatest among patients with recent bleeding events (70.1%) and stroke (63.7%) and lowest among those with recent myocardial infarction (54.9%). CONCLUSION: After the diagnosis of incident atrial fibrillation in older adults, mortality was the most frequent major outcome during the first 5 years. Among non-fatal cardiovascular events, heart failure was the most common event.