In a follow-up study, 77 patients with predominant mitral stenosis were examined to investigate the role of left atrial (LA) enlargement in LA thrombi. Fifteen (19.4%) patients had LA thrombi. Of these, 2 (13.3%) were in sinus rhythm and 13 (86.7%) in atrial fibrillation. Fourteen (93.3%) of the patients with LA thrombi had an LA size > or = 4.8 cm. Only one (6.7%) patient had an LA size of 4.4 cm and was in atrial fibrillation. The median LA size in patients with LA thrombi was 5.2 cm compared with 4.75 cm in patients without LA thrombi (p < 0.01). The relative risk for LA thrombi in patients with an LA size > or = 4.8 cm compared with patients with an LA size < 4.8 cm was 10.0 (95% confidence interval 1.4 to 71.4). It was thus confirmed that LA enlargement > or = 4.8 cm is an independent risk factor for LA thrombi in patients with mitral stenosis.