Specific information regarding the relation between infarct thickness and regional systolic function is important to the overall understanding of both the pathophysiology of acute and subacute myocardial infarction and the functional benefits of myocardial salvage interventions designed to limit the transmural extent of infarction and thereby preserve left ventricular function. In the present study, quantitative computer-assisted two-dimensional echocardiography was used to define the relation between infarct thickness and systolic function in the acutely and subacutely infarcted canine left ventricle. Echocardiograms were obtained at the mid-papillary muscle level at baseline and 6 h after occlusion (acute infarction) in eight animals and at baseline and 72 h after occlusion (subacute infarction) in nine animals. Systolic function was assessed by measuring the extent of fractional radial shortening along each of 36 evenly spaced endocardial targets from end-diastole to end-systole; the transmural extent of infarction was determined from the triphenyltetrazolium chloride-staining deficit at 6 and 72 h. The relation between systolic function and transmural extent of infarction was analyzed in two ways. First, the extent of fractional radial shortening in each group was examined as a function of quartile (25%) increments in transmural infarct thickness. This analysis revealed 1) a significant overall loss of fractional radial shortening with increasing transmural extent of infarction in both groups; and 2) significant differences in the extent of systolic dysfunction between successive quartile increments of infarction. Second, the relation between infarct thickness and systolic dysfunction was modeled mathematically by fitting the data from each infarct series to linear, logarithmic and exponential functions. This analysis revealed, for each animal tested, a significant downsloping linear relation between fractional shortening and transmural extent of infarction. Furthermore, the logarithmic and exponential curves generally modeled the data in a fashion indistinguishable from the linear function. Thus, this experimental study demonstrates for the first time a linear relation between the transmural extent of infarction and the extent of regional systolic dysfunction.