Epidemiologic data now indicate that atrial fibrillation (AF) is a common and serious condition associated with a 4-5 fold increased risk of stroke and a doubling of mortality. Data on the added cost imposed by AF are not available.Methods: One year mortality, stroke rates, in-hospital and out-patient costs were determined in a sample of hospitalized patients, aged 65-84 years, based on US Medicare data. Logistic and multiple linear regression analyses were done to take associated cardiovascular diseases (CVD) and length of follow-up into account in 12,625 patients with recent onset AF, compared to 13,809 patients without AF matched for age, sex and associated CVD.Results: 1-yr. mortality was increased by 25-70% and stroke occurrence from 1.65 to 2.26 fold in those with AF compared to comparable persons with CVD without AF. Total 1-year medical costs were increased by $2,500 (15.4%) in men with no age differential. In women, the cost was greater in those under age 75 (21.5% added cost) than above age 75. The added costs imposed by AF were almost entirely attributable to the in-hospital costs.Conclusions: AF is clearly not benign exacting a substantial toll in mortality, morbidity and medical costs above that of the associated medical conditions.