The risk for systemic embolization was studied in 272 patients without mitral stenosis or prosthetic valves who were referred to the echocardiography laboratory with atrial fibrillation (AF). During a mean follow-up period of 33 months (range less than 1 to 83), 27 (10%) patients had a systemic embolic event, which was cerebral in 23 patients (85%) and peripheral in 4 (15%). In the analysis of individual variables, the risk of embolization was increased by female sex, underlying heart disease and left atrial size greater than or equal to 4.0 cm, but not by age, hypertension or type of AF (paroxysmal vs chronic). In multivariable analysis, left atrial size greater than or equal to 4.0 cm was the single strongest predictor of increased risk for embolization (p less than 0.001), but female sex (p = 0.014) and underlying heart disease (p = 0.027) also contributed. When each of these 3 factors was assigned 1 point in a risk score, embolic events were found to occur in none (0%) of 24 patients with a risk score of 0, in 2 (3%) of 83 patients with a risk score of 1, in 13 (11%) of 118 patients with a risk score of 2 and in 12 (26%) of 47 patients with a risk score of 3. The score allows patients with AF and without mitral stenosis to be stratified into high-, medium- and low-risk groups for systemic embolization. Such information could be useful in decision making for anticoagulation in patients with AF.