The sizes of surgically induced acute myocardial infarctions were quantified in a study of 28 dogs. Four projections (right and left anterior oblique, anterior, and left lateral) were obtained with 129Cs myocardial scintigraphy. Control images, taken before surgery, were compared with images taken 24-72 hr after coronary artery ligation. From postmortem examination the size of the infarct was determined and expressed as a percentage of the total left ventricle. On a standard diagram four independent observers marked the infarcted areas in each projection, expressed the severity of involvement in each area, and determined overall infarction size as a percentage of the total left ventricle. A nonlinear least-squares method was also employed to derive the size of the infarct, using the results from each observer's diagram. There were positive correlations between each observer's percentage estimate and the autopsy results. The overall accuracy of the least-squares method was similar to that of the individual observers. In this study, Observer 3 proved that acute myocardial infarcts can be quantified accurately from multiple scintigraphic projections of the myocardium, but the other three observers had difficulty in estimating infarct size. This difficulty probably resulted from the lack of well-validated criteria to aid the observer in determining the area of infarction, the severity of involvement within that area, or the total size of a myocardial infarct. Improvement in these estimates will require the development of definitive criteria, the use of optical scanners or computer processing, and combinations of radionuclides.