This prospective study documents the natural history of the prehospital phase of 110 patients with acute myocardial infarction transported by a basic emergency medical system during a 22 month period. Ambulances in a mixed urban-rural county were staffed by basic emergency medical technicians certified in basic life support and the administration of intravenous fluids. Systolic blood pressure, pulse rate and cardiac rhythm were noted for all patients at the time of ambulance arrival and intermittently during transport. Analyses of patient data were performed to determine the relation between the occurrence of subsequent in-hospital urgent complications and death and 1) patient delay time, 2) initial pulse rate, 3) initial systolic blood pressure, and 4) initial cardiac rhythm. Twenty-three (21%) of the 110 patients died and 66 (60%) experienced at least one in-hospital urgent complication. When initial rhythm, pulse rate and blood pressure were considered, patients with hypotension had a higher mortality rate than did those who were either normotensive or hypertensive. The 10 patients with initial sinus bradycardia but no hypotension constituted a subgroup with zero mortality. These results identify high and low risk patient subgroups that may benefit from either providing or withholding interventions directed toward hemodynamic stabilization during the prehospital phase of acute myocardial infarction.