The echocardiographic features of left atrial ball thrombus associated with mitral stenosis are reviewed, and some previously unpublished cross-sectional echocardiographic findings presented. In one patient who had a large free-floating ball thrombus there was variation in its echocardiographic appearance; the thrombus was removed uneventfully at surgery. In another patient who had a pedunculated but immobile ball thrombus, a stalk was identified which attached it to the inter-atrial septum; this patient died suddenly before surgery could be performed, due to detachment of the thrombus and obstruction of the mitral valve orifice. Cross-sectional echocardiography is clearly superior to M-mode imaging in the detection of atrial thrombi, and variable appearances may help differentiation of thrombus from myxoma. Whether or not a ball thrombus appears mobile, emergency thrombectomy and mitral valve replacement is indicated, because of the risk of obstruction of the mitral valve.