To assess the clinical and prognostic importance of the early appearance of a functional left ventricular aneurysm after myocardial infarction, we used equilibrium radionuclide angiocardiography to study 51 patients with an initial anterior infarction. A functional aneurysm was defined as an area of systolic akinesis or dyskinesis with a distinct diastolic deformity and preserved adjacent wall motion. Functional aneurysms developed in 18 patients (Group 1). Left ventricular ejection fraction was comparable in this group and in the 33 patients without aneurysm (Group 2) (27.3 +/- 10 vs. 31.4 +/- 12 per cent). One-year mortality was markedly different, with 11 deaths (61 per cent) in Group 1 and 3 (9 per cent) in Group 2 (P less than 0.001). Six (55 per cent) of the deaths in Group 1 were sudden. Patients with a functional aneurysm appearing within 48 hours had the highest risk of dying (8 of 10). Thus, early formation of a functional aneurysm occurs frequently after anterior myocardial infarction and carries a high risk of death within one year that is independent of ejection fraction. In addition, the absence of a functional aneurysm identifies a large group with a low one-year mortality despite a markedly impaired ejection fraction.