In Phase I, the paramedics took electrocardiograms of patients who met the definition of cases and administered an abbreviated questionnaire. This phase tested the feasibility of paramedics making a diagnosis of acute myocardial infarction on the basis of clinical history and with the support of computerized electrocardiogram, coming to an appropriate decision to initiate thrombolytic therapy on the basis of the field database, and recognizing appropriate contraindications to thrombolytic therapy with and without remote physician supervision. An analysis was conducted of the pre-hospital selection of patients for thrombolytic therapy.