Resting tomographic myocardial perfusion images using technetium-99m-hexakis-2-methoxy-2-methylpropyl-isonitrile (Tc-Sestamibi) were obtained in 25 patients during their first myocardial infarction. Tc-Sestamibi was injected intravenously before acute reperfusion therapy, and repeated twice, at 18-48 hr, and at 6 to 14 days. Reperfusion was successful in 19 patients. In the patients with successful reperfusion, there was a mean decrease in the amount of hypoperfused myocardium between the initial and second studies (-9% +/- 12%, p = 0.004) and a further decrease between the second and final studies (-10% +/- 12%, p = 0.002). Nine of these 19 patients (47%) had evidence of significant improvement at the time of the second study. In six patients, significant improvement was not evident until the final study. Although tomographic imaging with Tc-Sestamibi following reperfusion therapy may show improvement in perfusion at 18-48 hr, the full extent of improvement is usually not evident until later.