There is a clear need to develop better measurement tools for assessment of the effect of acute therapy in myocardial infarction. Such tools must permit accurate measurement of the amount of myocardium that is initially at risk. 99mTc-sestamibi is a new radiopharmaceutical with unique properties that are well suited to the measurement of myocardium at risk without any delay in acute therapy. Tomographic imaging with this agent has been shown to provide accurate quantitative estimates of the myocardium at risk, which varies widely, even for patients with a similar coronary occlusion. For example, there is more than a threefold range in myocardium at risk for patients with a nonproximal occlusion of the left anterior descending coronary artery. The determination of the myocardium at risk before intervention and the change in this region after intervention constitute a promising measurement tool for the assessment of the effect of acute therapy. Initial studies with tomographic imaging have shown a significant improvement in a group of patients treated with thrombolysis, although the magnitude of improvement is highly variable in individual patients. Qualitatively similar results have been reported with planar imaging. Although it has a number of technical limitations, sequential imaging with 99mTc-sestamibi seems to have a clear advantage over those end points that have previously been used to assess acute therapy. This new measurement tool should facilitate future clinical trials of different treatments in acute myocardial infarction.