Background: This study examined the ability of dynamic 123I-labeled iodophenylpentade-canoic acid (IPPA) imaging to detect myocardial viability in patients with left ventricular (LV) dysfunction caused by coronary artery disease. Methods and Results: Serial 180-degree single-photon emission computed tomographic (SPECT) images (five sets, 8 minutes each) were obtained starting 4 minutes after injection of 2 to 6 mCi 123I at rest in 21 patients with LV dysfunction (ejection fraction [EF] 34%±11%). The segmental uptake was compared with that of rest-redistribution 201Tl images (20 segments/study). The number of perfusion defects (reversible and fixed) was similar by IPPA and thallium (11±5 vs 10±5 segments/patient; difference not significant). There was agreement between IPPA and thallium for presence or absence (κ=0.78±0.03) and nature (reversible, mild fixed, or severe fixed) of perfusion defects (κ=0.54±0.04). However, there were more reversible IPPA defects than reversible thallium defects (7±4 vs 3±4 segments/patient; p=0.001). In 14 patients the EF (by gated pool imaging) improved after coronary revascularization from 33%±11% to 39%±12% (p=0.002). The number of reversible IPPA defects was greater in the seven patients who had improvement in EF than in the patients without such improvement (10±4 vs 5±4 segments/patient; p=0.075). Conclusions: 123I-labeled IPPA SPECT imaging is a promising new technique for assessment of viability. Reversible defects predict recovery of LV dysfunction after coronary revascularization. © 1995 American Society of Nuclear Cardiology.