The effects of the extent of coronary collateral circulations, the duration of myocardial ischemia and recanalization of infarct-related vessels on left ventricular function were evaluated in 43 patients with acute anteroseptal myocardial infarction. All patients had complete occlusions of their proximal left anterior descending coronary arteries and were treated with intra-coronary thrombolytic therapy within 8 hours after the onset of their chest pain. The 43 patients were categorized in 4 groups based on the extent of their coronary collaterals in the early period of myocardial infarction and the results of thrombolysis. Group A consisted of 11 patients with well-developed collaterals who had successful thrombolysis. Group B was comprised of 14 patients with poorly developed or no collaterals, and successful thrombolysis. In group C, there were 9 patients with well-developed collaterals and unsuccessful thrombolysis. In group D, there were 9 patients who had poorly or not developed collaterals, and all had unsuccessful thrombolysis. Four weeks after the intervention, ejection fraction (EF) and regional wall motion (RWM) were calculated from the data of the left ventricular angiograms. There was no significant difference in patients' age, sex, nor in peak serum creatine kinase among the 4 groups or the duration of myocardial ischemia between groups A and B. Patients with successful thrombolysis (groups A and B) had significantly higher EF and preserved RWM of infarct areas compared to patients with unsuccessful thrombolysis (groups C and D, p less than 0.05). Thirteen patients with early reperfusion (within 4 hours after the onset of chest pain) had significantly higher EF and better RWM than did 12 patients with late reperfusion and 18 patients with unsuccessful thrombolysis (p less than 0.01). However, there was no significant correlation between the duration of myocardial ischemia and RWM of the infarct areas among 25 patients who had successful thrombolysis (r = -0.3, NS). Patients in group A had higher EF and better RWM of infarct areas than did patients in groups B, C and D (p less than 0.01). In addition, 3 patients with well-developed collaterals had good RWM despite late reperfusion which occurred more than 4 hours after the onset of symptoms. These results suggest that the extent of coronary collaterals during the early period of myocardial infarction and the time delay from the onset of symptoms to the initiation of thrombolytic therapy are important factors for the salvage of left ventricular function in patients with myocardial infarction.