The value and limitations of stress 201T1 myocardial single photon emission computed tomography (SPECT) for diagnosing ischemic heart disease (IHD) was studied. Using a dual-head rotating gamma camera system, stress SPECT and conventional planar imaging were performed for 138 patients while they were examined by symptom-limited graded bicycle ergometer exercise. All patients underwent selective coronary arteriography and left ventriculography, and 93 had myocardial infarction (MI), 30 had effort angina (EA) and 15 were normal (control). Sensitivities for detecting IHD (SPECT: planar = 96%: 89%, p less than 0.01), individual coronary arterial lesions (left anterior descending artery = LAD, 84%: 68%, p less than 0.005; left circumflex artery = LCX, 60%: 47%, NS; right coronary artery = RCA, 88%: 69%, p less than 0.01), multivessel disease (= LAD + LCX and/or RCA, 53%: 31%, p less than 0.025), and three vessel disease (60%: 13%, p less than 0.005) were significantly higher by SPECT than by planar imaging. In addition, detection of ventricular aneurysms by SPECT was possible with a reasonably high sensitivity (94%) and specificity (84%). Signs of aneurysm included 1) an extensive anterior permanent defect, 2) a large left ventricular cavity, and 3) widening of the angle composed by the septal and lateral walls toward the apex in transaxial images. Sensitivity for detecting IHD was significantly lower in patients without MI (i.e., EA) than in patients with MI (MI: EA = 100%: 83%, p less than 0.005). Sensitivity for detecting individual coronary arterial lesions was lower in the absence than in the presence of MI (LAD; 77%: 87%, LCX; 38%: 68%, RCA; 71%: 90%, respectively), with multivessel disease than with single vessel disease, and with mild than with severe grade of stenosis. Sensitivity for detecting multivessel disease was lower in patients without MI than in those with MI (31%: 61%, respectively), and in anterior MI than in posteroinferior MI, or both MIs (36%: 69%: 100%, respectively). Stress-induced ischemia of infarcted area (anterior MI, 36%; posteroinferior MI, 24%) and ventricular aneurysm (anterior MI, 21%; posteroinferior MI, 0) masked other coronary arterial stenoses in patients with previous MI. We concluded that stress 201T1 myocardial SPECT was a useful non-invasive technique for detecting IHD and individual coronary arterial lesions, multivessel disease (especially posteroinferior MI and anterior + posteroinferior MI), three vessel disease and ventricular aneurysms. However, there were limitations in detecting multivessel disease in patients with anterior MI and EA.