OBJECTIVE Prospective evaluation of the effects of the intravenous administration of rt-PA (Alteplase) up to 6 hours after the pain onset on the patency of the AMI related artery, mortality, adverse reactions and complications. METHODS Open, multicenter, non-comparative study involving 139 patients with diagnosis of AMI, with less than 6h of duration. The rt-PA was intravenously administered, in a dose of 100mg, as follows: 10mg in the 1st 2min, 50mg in 58min and 40mg in 120min. In addition, the patients received intravenous heparin (5000 IU at first and then, 1000 IU/hour, for 24h), aspirin (500mg in the 1st day and then, 100mg/day) and dipyridamole (75mg, three times a day), during the hospitalization period. The angiographic study was performed in 129 (93%) patients, within the 1st week of AMI. RESULTS The age of the patients ranged from 29 to 85 (mean 56.6 +/- 10.3) years. The related artery for the AMI was patent (TIMI II and III flow) in 92/129 (71%) patients, with a mean ejection fraction of 50 +/- 14%, a value higher than that exhibited by patients with TIMI 0 and I flow (average ejection fraction = 44 +/- 14%). Reinfarction was diagnosed in 9 (6.4%) patients during the hospitalization period. During this period, there were 9 (6.4%) deaths. Minor hemorrhages were observed in 19 (12%) patients and major hemorrhages in 3 (2%) cases. No patient experienced stroke. CONCLUSIONS The administration of the rt-PA therapy in the AMI was associated to a high reperfusion index of the related artery for the infarction, with improved left ventricular function and low incidence of reinfarction and in-hospital mortality, as well as, complications.