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.