The effectiveness of thrombolytics has been clearly demonstrated in more than half the cases in the large cohorts of patients selected for trials during the acute phase of myocardial infarction. At individual level, thrombolysis will clinically either succeed or fail so, for the medical team managing the patient, choice of treatment may be likened to a gamble which in the best of cases (most often) leads to an uncomplicated success and, in the worst of cases, failure worsened by a severe complication. OPTIMAL is a multidisciplinary and multicentre, prospective cohort study associating mobile medical teams and interventional cardiology units to test the hypothesis that the outcome of prehospital thrombolysis does not depend on chance alone but also varies according to demographic, etiological, clinical and logistic factors involved in the occurrence and management of myocardial infarction. The primary objective of this French study, conducted over one year on more than 800 subjects, is to identify the predictors of the results of prehospital thrombolysis from a very early angiographic evaluation. The results for this cohort may be useful for setting up appropriate management strategies for acute myocardial infarction, from the prehospital phase (thrombolysis or not) up to in-hospital orientation of the patients (angiography room or Intensive Care Unit) and to determine the most judicious time for coronary angiography. OPTIMAL is to date the largest prospective serie of prehospital thrombolysis evaluated by an early angiographic control.