Regional and global left ventricular performance was noninvasively assessed with quantitative gated equilibrium radionuclide ventriculography in 43 patients an average of 40 hours after the onset of a first acute transmural myocardial infarction. In all 16 patients with anterior infarction, regional ejection fraction, a quantitative measure of regional left ventricular performance, was uniformly depressed in the infarcted zone. In patients with inferior infarction the abnormalities of regional performance were less severe. Fourteen of 20 patients (70 percent) with inferior infarction had depressed performance in the infarcted zone. Function in noninfarcted zones was abnormal in only 6 of the 20 patients (30 percent) with inferior infarction, but it was abnormal in 11 of the 16 patients (69 percent) with anterior infarction, particularly in those with severe pump failure. As a consequence, global left ventricular ejection fraction was significantly lower in patients with anterior than in those with inferior infarction (mean +/- standard error of the mean 31 +/- 3 percent versus 51 +/- 3 percent, less than 0.005). Prognosis and clinical functional class were related to performance not only in infarcted zones, but also in noninfarcted zones as assessed with electrocardiography. It is concluded that depressed function in apparently noninfarcted left ventricular zones contributes significantly to left ventricular dysfunction after acute myocardial infarction, particularly in patients with anterior infarction.