Twenty-five patients with an anterior wall acute myocardial infarction (AMI) were studied by 2-dimensional echocardiography (2-D echo) 3 to 5 days after the onset of chest pain, and serially over 3 to 24 months to determine if a particular pattern of contractility may predispose to left ventricular (LV) aneurysm formation. No subject had a prior AMI. In 8 subjects LV aneurysm eventually developed (group I), usually within 2 to 4 weeks of AMI; in 17 patients LV aneurysm did not develop (group II). Percent fractional shortening of the basal and midventricular segments was significantly better in group I subjects than in group II subjects (29 +/- 2% vs 20 +/- 2%, p less than 0.01, respectively, for the basal segment, and 23 +/- 1% vs 17 +/- 2%, p less than 0.02, respectively, for the midventricular segment). Infarct size as determined by peak creatine kinase isoenzyme levels was large in both groups, and there was no statistically significant difference between their mean values (2,099 +/- 620 IU vs 1,334 +/- 249 IU for groups I and II, respectively). Severe asynergy of the infarcted myocardium was present in all group I subjects and in 9 of 17 group II subjects on the initial 2-dimensional echocardiographic study. These results indicate that LV aneurysm formation depends on a critical imbalance of myocardial forces where strong LV segments cause bulging of weakened ones.