The left atrium has been opened to reveal the most common primary cardiac neoplasm--an atrial myxoma. These benign masses are most often attached to the atrial wall, but can arise on a valve or in a ventricle. They can produce a "ball valve" effect by intermittently occluding the atrioventricular valve orifice. Embolization of fragments of tumor may also occur. Myxomas are easily diagnosed by echocardiography. They may also be seen with chest CT scan and with magnetic resonance angiography.