Myocardial infarct size is one of the most important predictors of prognosis in patients suffering an acute myocardial infarction. It can be assessed by enzymatic and electrocardiographic methods. The present report compares dynamic vectorcardiographic monitoring, serial plasma enzyme activity measurements and QRS scoring according to Palmeri as techniques for infarct size estimation. We report the results from 74 patients with acute myocardial infarction, who participated in a randomized trial of treatment with alteplase. A good correlation was found between myocardial infarct size by estimation from enzymatic measurement and from dynamic vectorcardiography. Dynamic vectorcardiography correlated more closely with enzymatically estimated infarct size in patients with Q-wave infarction, regardless of infarct location, than did QRS scoring of the conventional 12-lead electrocardiogram. Furthermore, dynamic vectorcardiography requires no time-consuming analysis and can be used for on-line monitoring of patients with ongoing infarction to estimate the size of an acute infarction while it is developing.