Mobile masses within the left atrial cavity are commonly caused by organized thrombi or left atrial myxoma. The use of transesophageal echocardiography has provided means for differentiation between the two conditions. We report a case of a left atrial mass in a middle age male patient having past history of closed mitral commissurotomy presented with mitral stenosis and atrial fibrillation. On transthoracic echocardiography a round mass appeared freely mobile in left atrium but did not prolapse between the mitral leaflets due to the tightly stenosed valve orifice. In the presence of mitral stenosis, atrial fibrillation and dilated left atrium, the appearance of the mass was in keeping with mobile thrombus but possibility of left atrial myxoma cannot be ignored. Consequently, the patient was referred for surgical treatment and the mass removed through left atriotomy. Microscopic examination revealed a highly organized thrombus. We conclude that TTE is still a reliable tool in the diagnosis of large mobile atrial thrombi; TEE may help to differentiate clot from atrial myxoma but histopathological examination is a gold standard.