Quantification of myocardial infarct (MI) size is of prognostic importance in patients with acute ischemic damage. Evaluation of the efficacy of interventions for salvage of ischemic myocardium depends on the accurate estimation of the ischemic area and a knowledge of the natural progression of the infarct. Computerized transmission tomography (CTT) is a reliable in vivo technique for estimating infarct size. We serially studied 8 dogs over approximately 1 month after occlusion of the left anterior descending coronary artery using both ungated and prospectively electrocardiogram-gated CTT. Scans were obtained 20 minutes after occlusion and then several more times until the dogs were killed. Using the ungated CTT scans, infarct size increased from 0 to 4 days (+ 65 +/- 20%, mean +/- standard error of the mean, p less than 0.05), then progressively decreased. The initial perfusion defect overestimated the eventual MI size at 1 month by 33 +/- 15% (p less than 0.05). The MI size at necropsy correlated well (r = 0.98, p less than 0.001) with CTT MI size determined just before sacrifice. Non-infarcted left ventricular (LV) muscle mass increased significantly (27 +/- 7% greater at 1 month compared with day 0, p less than 0.01) over time, presumably representing compensatory LV hypertrophy. The LV muscle mass at necropsy correlated well (r = 0.94, p less than 0.001) with CTT LV muscle mass just before sacrifice.(ABSTRACT TRUNCATED AT 250 WORDS)