This study tested the hypothesis that reperfusion therapy might provide benefit at two levels: 1) by arresting infarct migration at the endocardial level, such that partial or complete recovery of regional function occurs; and 2) if the former is not achieved, by preventing complete or near complete transmural migration and subsequent infarct expansion. To test this hypothesis, 24 patients who received intravenous streptokinase therapy within 4 h of chest pain were studied prospectively. All patients underwent two-dimensional echocardiography at the time of admission and 1, 2, 3 and 10 days later. The patients also underwent coronary angiography 2 h after completion of streptokinase therapy. Although 18 (75%) of the 24 patients had a patent infarct-related artery, only 8 (45%) of the 18 patients with this finding showed improvement in regional function. Improvement was not evident until 3 to 10 days after streptokinase therapy. In addition to the presence of an open infarct-related artery, the interval between chest pain and onset of streptokinase therapy (2.5 +/- 0.5 versus 3.2 +/- 0.7 h, p = 0.02) differed significantly between patients who did or did not show improved regional function. Of the 15 of 16 patients with no improvement in regional function, 4 showed infarct expansion, and all had a closed infarct-related artery compared with only 2 of the 11 not showing expansion (p = 0.01). In conclusion, intravenous streptokinase given within 4 h of chest pain results in improvement in regional function in about 33% of the patients, presumably by arresting the infarction within the endocardium.(ABSTRACT TRUNCATED AT 250 WORDS)