The 99mTc-labeled isonitriles are a new class of myocardial imaging agents that, because of superior physical characteristics, may be better for myocardial perfusion imaging than 201Tl, particularly with SPECT imaging modalities. The initial experimental and clinical experience with one of these radiotracers, 99mTc-MIBI, suggests that this radiotracer may be promising for the assessment of myocardial ischemia and infarction. The capability of assessing risk area during acute coronary occlusion and extent of myocardial salvage after reperfusion offers a noninvasive means to evaluate the efficacy of therapeutic interventions aimed at reducing ischemic myocardial injury. 99mTc-MIBI will provide a unique noninvasive means of simultaneously assessing myocardial perfusion and function, ultimately permitting a better understanding of ischemic-induced myocardial dysfunction. The analysis of the myocardial perfusion and function relationship may possibly provide a method of discriminating reversible myocardial ischemia from irreversible myocardial injury. Algorithms will need to be developed for the optimal quantification of 99mTc-MIBI images. Additionally, single-day protocols using split doses of 99mTc-MIBI will need to be further developed before 99mTc-MIBI imaging can be widely employed. Although further clinical studies are necessary in larger numbers of patients before the full clinical implications of this radiotracer are known, the initial experience is encouraging.