In order to evaluate the usefulness of thallium-201 (201Tl) myocardial scintigraphy in delineating the location and size of prior myocardial infarction, 32 patients were evaluated at a mean of 7 ± 2 months after infarction with a 12-lead ECG, resting 201Tl myocardial scintigram, biplane left ventriculogram and coronary angiograms. From the left ventriculogram, asynergy was quantified as percent abnormally contracting segment (% ACS), the percent of end-diastolic circumference which was either akinetic or dyskinetic. Using a computerized planimetry system, we expressed 201Tl perfusion defects as a percentage of total potential thallium uptake. Of 21 patients with ECG evidence of prior transmural infarction, a 201Tl defect was present in 20 (95%), and angiographic asynergy was present in all 21 (100%). The site of prior infarction by ECG agreed with the 201Tl defect location in 24 of 32 patients (75%) and with site of angiographic asynergy in 23 of 32 patients (72%). Scintigraphic defects were present in only four of 10 patients (40%) with ACS ≤ 6%, but scintigraphic defects were found in 20 of 22 patients (91%) with ACS > 6% (p < 0.01). Thallium defect size correlated marginally with angiographic left ventricular ejection fraction (r = - 0.60) but correlated closely with angiographic % ACS (r = 0.80). Thallium defect size was similar among patients with one-, two, or three-vessel coronary artery disease (≥ 70% stenosis), but thallium defect size was larger in patients with electrocardiographic evidence of transmural infarction (p < 0.01) or pulmonary capillary wedge pressure > 12 mm Hg (p < 0.001). Thus, resting 201Tl myocardial scintigraphy is useful in localizing and quantifying the extent of prior myocardial infarction, but is insensitive to small infarcts (ACS < 6%).