In order to correlate left atrial diameter (LAD) with the prevalence of systemic embolism (SE) in mitral stenosis (MS), we assessed LAD by M mode tracings in 51 patients with SE and in 50 patients with MS without ES as control group (C). Mean age was similar in both groups (SE 47.3 +/- 12 vs C 46.8 +/- 14 years; p NS) (mean +/- SD). Functional class, cardiothoracic ratio and association of other valvular lesions were similar in both groups. Atrial fibrillation (AF) was more frequent in SE group (n = 39) than in C group (n = 20) (p less than 0.01). LAD in SE patients ranged from 2.9 to 9 cm (5.2 +/- 1) whereas in C patients range was from 2.8 to 7.5 (4.6 +/- 1) (p less than 0.01). Nevertheless, LAD in patients with AF was rather similar in both groups (SE 5.3 +/- 1.1 vs C 5.3 +/- 1; p NS). Our results suggest that LAD is not a good predictive parameter for SE in MS. The main risk factor for SE was the existence of AF. Echocardiographic LAD is not a useful parameter to prescribe chronic oral anticoagulation as prophylaxis for SE in patients with MS.