In order to evaluate the usefulness of thallium-201 (201TI) 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 201TI 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 201TI perfusion defects as a percentage of total potential thallium uptake. Of 21 patients with ECG evidence of prior transmural infarction, a 201TI 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 201TI 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 less than or equal to 6%, but scintigraphic defects were found in 20 to 22 patients (91%) with ACS greater than 6% (p less than 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 (greater than or equal to 70% stenosis), but thallium defect size was larger in patients with electrocardiographic evidence of transmural infarction (p less than 0.01) or pulmonary capillary wedge pressure greater than 12 mm Hg (p less than 0.001). Thus, resting 201TI myocardial scingigraphy is useful in localizing and quantifying the extent of prior myocardial infarction, but is insensitive to small infarcts (ACS less than 6%).