A method has been developed for measurement of myocardial infarct size from thallium-201 scintigrams that depends on computer measurement of levels of radioactivity in the myocardium. In 16 dogs, thallium-201 scintigrams were obtained in the left lateral and left anterior oblique projections 48 hours after ligation of the left anterior descending coronary artery. Scintigraphic results were obtained by two independent observers and were compared with tissue measurements of infarct volume calculated from thallium autoradiograms and nitro-blue tetrazolium (NBT)-stained tissue slices. Infarct volumes derived from tissue measurements were used to develop criteria for the computer scintigraphic technique. There was no significant difference in the scintigraphic measurements made by the two observers. Scintigraphic infarct size in the left lateral and left anterior oblique projections correlated with tissue infarct size with r values of 0.88 and 0.75, respectively, for thallium autoradiography and 0.71 and 0.70, respectively, for NBT tissue staining. The range of infarct volume was 3.3 to 14.8 percent of the left ventricular mass. Results of this study suggest that scintigraphic quantitation of infarct size is feasible in this dog model.