Four patients are presented in whom either a false aneurysm or a "pseudo-false" aneurysm of the left ventricle developed following a myocardial infarction. False aneurysms of the left ventricle are unusual and are distinctly different from the more common true aneurysms. A false aneurysm is the result of a contained hematoma dissecting, into a transmural infarct. It communicates with the left ventricle through a small orifice. Previous descriptions of false aneurysms have stressed that their wall consists of pericardium and mural thrombus and lack identifiable epicardial or myocardial elements. Two pseudo-false aneurysms are described. They communicated with the left ventricle through a small orifice but their wall contained myocardial tissue. False aneurysms have a tendency to rupture and therefore their presence alone is an indication for operation. One of the pseudo-false aneurysms discussed ruptured into the right ventricle. The operation for false aneurysm may be simpler than that for true aneurysm since it might be possible to close the small communication into the left ventricle without resecting the entire aneurysm wall.