The primary goal of this study was to assess the frequency of persistent regional wall motion abnormalities and myocardial perfusion defects detectable late after myocardial infarction with radionuclide ventriculography and thallium-201 imaging, respectively. The study was performed prospectively in 32 patients in whom infarct size was estimated enzymatically at the time of the acute episode and in 10 patients without infarction. Thallium-201 imaging and radionuclide ventriculography were performed with the patient at rest an average of 11 months after infarction (range 6 to 20 months) and analyzed independently by two observers who were unaware of results of other clinical and laboratory data. Perfusion defects were detected in 94 percent (30 of 32) by observer I and in 91 percent (29 of 32) by observer II. Wall motion abnormalities were detected in 78 percent (25 of 32) and 75 percent (24 of 32) by observers I and II, respectively, but in 10 of the patients with an infarct size less than 20 creatine kinase-gram-equivalents (CK-g-eq), wall motion abnormalities were found in only 50 and 40 percent, respectively, by these observers (p <0.04). Electrocardiographic changes of infarction (Q waves) were present in only 56 percent (18 of 32) of patients. Sixty-eight percent of patients with an infarct size greater than 20 CK-g-eq had persistent Q waves but these were present in only 30 percent with an infarct size less than 20 CK-g-eq (p <0.04). The sites of perfusion defects and of wall motion abnormalities corresponded closely and were concordant with electrocardiographic localization. Thus, thallium-201 imaging and radionuclide ventriculography are sensitive noninvasive techniques for identifying prior myocardial infarction, and are particularly helpful in patients with conduction abnormalities or equivocal electrocardiographic findings.