This study examined the importance of viability as a clinical issue in 532 patients with angiographically proven CAD who underwent exercise SPECT thallium imaging. Conventional 4-hour delayed images were used to differentiate scar tissue from ischemia (20 segments per patient). There were 90 patients (17%) with normal images, 274 patients (52%) with reversible defects only, and 168 patients (31%) with scar tissue either with or without associated ischemia. The patients with scar tissue were subdivided according to the number of segments with fixed defects and the number of additional reversible defects. There were 114 patients with scar tissue alone or more scar tissue than ischemia. Contrast ventriculography in these 114 patients revealed normal wall motion or ejection fraction in 50 patients. On the basis of results of thallium imaging alone, the issue of viability was probably significant in 114 patients (21%); however, when the ventriculographic data were also included, the issue was significant in only 64 patients (12%) (p < 0.001). Thus myocardial viability is an important issue in 21% of patients with CAD when conventional thallium imaging is used, but this percentage decreases to 12% when wall motion and ejection fraction data are also included. These data may be important in considerations for the need of metabolic imaging and emerging scintigraphic techniques. © 1992.