Although M-mode echocardiography (MME) is not a reliable method for detecting left atrial thrombi, recent reports suggest that two-dimensional echo (2DE) may be more effective than MME in identifying intracardiac thrombi. In three patients with prosthetic mitral valves who presented with either arterial embolization or prosthetic valvular dysfunction, 2DE demonstrated left atrial masses consistent with thrombi, while MME was either negative (two patients) or suspicious (one patient) for left atrial thrombus. Thrombi were documented by surgical or postmortem examination in all cases. Clear delineation of the atrial cavity and the margins of the masses, visualization on multiple echocardiographic views, and comparison of serial examinations were helpful in identifying these masses as thrombi. In addition, the masses visualized had certain patterns of motion which seem unique and may allow characterization of atrial masses as thrombi.