Thirty-nine patients with severe cardiac failure and a previous transmural myocardial infarct were studied prospectively to assess the ability of M-mode echocardiography to detect resectable left ventricular aneurysms. Subsequent angiography showed an aneurysm in 23 patients and diffuse left ventricular damage in 16. Nine (57%) of the patients without aneurysm had either left ventricular end-systolic dimensions in excess of 6 cm combined with reduced contraction of the base of the left ventricle (fractional shortening of the level of the mitral valve less than 20%), or gross reduction of contraction (fractional shortening less than 15%) regardless of left ventricular dimension. These abnormalities did not occur in any patient with an aneurysm. In 21 (91%) of the patients with a left ventricular aneurysm its presence was correctly predicted either by fractional shortening in excess of 20% (16 patients) or gross delay of mitral valve opening (greater than 80 ms 16 patients). In one patient without an aneurysm, fractional shortening was greater than 20% and thus incorrectly suggested the presence of aneurysm. Only 39% of aneurysms were directly visualized. Therefore M-mode echocardiography can correctly rule out the diagnosis in over 50% of patients without an aneurysm and strongly suggest the possibility in over 90% of patients with one.