Thirty patients with recent myocardial infarction were intravenously injected with the modified fatty acid [123I]16-iodo-3R,S-methyl hexadecanoic acid (MIHA) at peak exercise to quantify viable myocardium after infarction. The results were analysed visually and quantitatively and compared with those obtained after injection of thallium 201 at peak exercise with single-photon emission computed tomography (SPECT) imaging immediately and 4 hours later. Ventriculography was used to study regional wall motion in all patients. In the visual comparison of 201Tl- and MIHA-SPECT scintigrams, 98.8% of normal segments (N) and 96% of temporary thallium-defect segments (T) were N or T on MIHA-SPECT scintigrams (91.3% and 70.5%, respectively, in the quantitative analyses). In contrast, 47.2% of permanent thallium-defect segments (P) were N or T on the MIHA-SPECT scintigrams (20.6% in the quantitative analysis). Revascularization therapy could only be recommended on the basis of MIHA-SPECTs in 5 to 8 of the 30 patients. To confirm the superiority of MIHA over 201Tl to evaluate myocardial viability, one must compare their uptake with myocardial contractility after revascularization.