Although the efficacy of intravenous thrombolysis in the treatment of acute myocardial infarction has been widely proved, some uncertainty concerning the "temporal window" of administration still persists. The aim of the present investigation was to study whether the late administration of a thrombolytic agent (6 or more hours after the onset of symptoms of acute myocardial infarction) offers any short or long-term advantages with regards to left ventricular function and clinical outcome. We studied 100 consecutive patients at their first episode of myocardial infarction, admitted to Coronary Unit within 24 hours of the onset of symptoms. Of these patients, 62 were administered rt-PA (44 patients within the 6th hour, and 18 between the 6th and 24th hour after the onset of symptoms) and the 38 remaining patients, who did not receive the thrombolytic agent (due to concerns with respect to possible complications), constituted the control group (18 admitted within 6 hours and 20 between 6 and 24 hours). All patients underwent serial electrocardiograms, and echocardiograms upon admission and at discharge to assess the ejection fraction, the asynergy score and the percentage of ischemic area. Furthermore, the survivors were invited for a follow-up examination one year after their acute initial episode. Seven cases of heart failure occurred, before discharge, among the control patients admitted 6 to 24 hours after onset of symptoms, compared with no cases in the subgroup of patients treated with rt-PA during the same time period (p = 0.0068).(ABSTRACT TRUNCATED AT 250 WORDS)