Electrocardiographic (ECG) evidence of cardiac ischemia or infarction is difficult to detect in the presence of left bundle branch block (LBBB). Traditional ECG indicators of ischemia, such as ST- segment elevation, are common in LBBB and may not indicate acute ischemia. Proper evaluation of the initial ECG is crucial in selecting candidates for early thrombolysis, because the earlier reperfusion treatment is administred, the better are the results. Individuals with LBBB are particularly important stratum of patients to identify. This is true not only because they have a high baseline mortality and receive the greatest incremental improvement in survival when given thrombolytic agents but also because it is tendency to undertreat them. The criteria of Sgarbosa are too insensitive to be used as screening (roule out) test to determine which patients with an LBBB do not have an AMI. The Sgarbosa criteria are, however, highly specific and can be used reliably as confirmatory test to rule in AMI in patients with LBBB. ECG alone doesn't support the diagnosis of AMI. Elevated value of biochemical markers of myocardial necrosis in the presence of LBBB confirms the diagnosis. Despite the recently updated joint practical guidelines of American Heart Association (AHA) and American College of Cardiology (ACC) which defines that all patients having symptoms consistent with acute MI and LBBB should be treated like ST-segment elevation, only minority of them receive thrombolytic therapy, particularly the elderly (only 4%). In the absence of definitive diagnosis of AMI doctors withhold from decision to administer thrombolytic treatment because of risk of haemorrhagic complications. There are not perfect diagnostic tools allowing early diagnostic of AMI in patients having LBBB. Currently the best justified strategy is to follow AHA/ACC recommended guidelines to administer thrombolysis to all patients with LBBB presenting with chest pain, particularly if serum biomarkers are elevated.