OBJECTIVES We sought to determine the relationship between clinical risk factors for systemic thromboembolism in patients with atrial fibrillation and the prevalence of left atrial (LA) spontaneous echocontrast (SEC) and LA thrombus (LAT). BACKGROUND Atrial fibrillation is associated with an increased risk of systemic thromboembolism. LA SEC and LAT also predict thromboembolic events. The relationship between clinical risk factors for systemic thromboembolism and prevalence of LA SEC and LAT is unknown. METHODS In all, 524 patients with atrial fibrillation underwent transesophageal echocardiography between August 2000 and March 2005. Clinical risk factors for systemic thromboembolism were determined for each patient. A CHADS(2) score ranging from 0 to 6 was calculated for each patient as: congestive heart failure = 1 point; hypertension = 1 point; age 75 years or older = 1 point; diabetes mellitus = 1 point; and history of stroke including transient ischemic attack or systemic embolism = 2 points. Transesophageal echocardiography reports were reviewed for the presence of LA SEC and LAT. Univariate and multivariable models were structured to assess which clinical risk factors predicted the presence of LA SEC or LAT. RESULTS In a multivariable model, age 75 years or older, previous thromboembolic event, and left ventricular ejection fraction (LVEF) less than 40% predicted LA SEC, whereas LVEF less than 40% was the only predictor of LAT. LA SEC was present in 24% of patients with a CHADS(2) score of 0, but was present in 58% with a CHADS(2) score of 5 or 6 (P < .0001). LAT was present in 3% percent of patients with a CHADS(2) score of 0, but in 17% of patients with a CHADS(2) score of 5 or 6 (P = .0026). CONCLUSION Age 75 years or older, previous thromboembolic event, and LVEF less than 40% predict presence of LA SEC. LVEF less than 40% is the only multivariate predictor of LAT. The prevalence of LA SEC and LAT increases with increasing CHADS(2) score.