BACKGROUND: Thromboembolism related to atrial fibrillation (AF) is a major cause of morbidity and mortality. Patients with acute thromboembolism and AF are at high risk for early recurrent events. OBJECTIVE: To determine the prevalence of left atrial thrombi in patients who had acute thromboembolism and newly diagnosed AF. PATIENTS AND METHODS: Adult inpatients with AF were screened to identify those with acute (< 36 hours) systemic thromboembolism and newly recognized AF. Of 41 qualifying patients, 31 (76%) agreed to undergo transesophageal echocardiographic study, including 24 with acute neurologic events and seven with peripheral thromboembolism. A control population consisted of 88 adults with newly recognized AF without clinical thromboembolism. RESULTS: Transesophageal echocardiography identified left atrial thrombi in 13 (43%) of the 30 study patients who underwent transesophageal echocardiography compared with nine (10%) of 87 controls (P < .001). Spontaneous echo contrast was identified in 27 (87%) of the study population vs 42 (48%) of controls (P < .001). The prevalence of this marker of blood stasis did not differ between patients with left atrial thrombi without thromboembolism (P = .69). Duration of AF, prevalence of abnormal left ventricular function, left atrial size, and mitral regurgitation were similar in both groups. CONCLUSIONS: Left atrial thrombi were identified in more than 40% of patients with acute thromboembolism and newly recognized AF. These data suggest that a major source of recurrent thromboembolism in this group may be residual thrombus migration. Among patients with AF and atrial thrombi, clinical thromboembolism seems to occur randomly, or is related to an unidentified process.