OBJECTIVE Late thrombolytic treatment (after 6 hours from pain onset) for acute myocardial infarction has been shown to improve survival in a cohort of patients. The mechanisms underlying such benefit have been debated and a controversy exists about the influence of late thrombolysis over left ventricular function as one potential mechanism. The present study intended to clarify the effects of late thrombolysis with alteplase on left ventricular function parameters. METHODS Prospective, multicenter, randomised, double-blind study. Ancillary study to LATE trial. METHODS We studied 103 patients, 81.6% men, randomly allocated to alteplase (100 mg over 3 hours) or placebo 6 to 24 hours after onset of myocardial infarction. METHODS Left ventricular function parameters by radionuclide ventriculography were evaluated at 1 and 6 months after myocardial infarction. RESULTS At 1 month, left ventricular ejection fraction was 48.86% on alteplase and 43.19% in placebo group (p = 0.028), with greater benefits for septal and apical regions. At 6 months no significant differences were found. CONCLUSIONS Those data suggest that late thrombolysis with alteplase improves left ventricular function 1 month after myocardial infarction although the difference did not persist at 6 months. Improvement of left ventricular function shall, therefore, be considered as one of the possible mechanisms underlying the benefit of late thrombolysis.