Among a group of 18 patients with severe angina and an ejection fraction of less than 46%, cine-angiography was performed on 18, 2-D echocardiography on 18, radionuclide ventriculography on 13 and 201Thallium myocardial scintigraphy on 13 at rest, both pre-operatively and six months after an aortocoronary bypass operation. In nine patients there was a significant increase postoperatively in the ejection fraction (P less than 0.001) (group I), while it remained practically unchanged in the remainder (group II). Similar results were obtained with radionuclide ventriculography and echocardiography. Ischaemia at rest was demonstrated by myocardial scintigraphy preoperatively in an average of 10 out of 27 sectors in group I patients, but within six months postoperatively decreased to 3.1 (P less than 0.01). In group II patients the mean number of sectors with ischaemia at rest was pre-operatively 2.5, postoperatively 6.0. With one exception there was an improvement in left-ventricular function at rest in all patients with pre-operatively 7 or more sectors showing ischaemia at rest. These results suggest that myocardial scintigraphy makes it possible to identify those patients with severe ischaemic myocardial damage who would profit from revascularization.