Substantial time delays from symptom onset to diagnosis and treatment of patients with acute myocardial infarction have been demonstrated. To determine the relative importance of prehospital mode of patient transport and the relative impact of emergency medical system transport with or without a prehospital cellular electrocardiogram (ECG) on hospital time delays to initiation of thrombolytic therapy, four prospective parallel groups of patients with acute myocardial infarction were evaluated. The median hospital time delay to treatment median (twenty-fifth and seventy-fifth percentiles) was 64 minutes (46 and 87 minutes, respectively, for twenty-fifth and seventy-fifth percentiles) for patients transported by private automobile ("walk-in"); 55 minutes (45 and 68 minutes, respectively) for patients transported by local ambulance; 50 minutes (38 and 81 minutes, respectively) for patients transported by the emergency medical system without a prehospital ECG; and 30 minutes (27 and 35 minutes, respectively) for patients transported by the emergency medical system who had a 12-lead ECG transmitted from the field. Patients transported by the emergency medical system were randomized to receive cellular telephone transmission of a prehospital 12-lead ECG. Specialized emergency medical system transport alone did not facilitate in-hospital initiation of thrombolytic therapy in patients with acute myocardial infarction when compared with those brought by local ambulance or by private automobile. A significant reduction in hospital time delay to treatment was observed only in patients transported by the emergency medical system who had cellular transmission of a prehospital 12-lead ECG from the field.