To assess the impact of a field-transmitted electrocardiogram (ECG) on patients with possible acute myocardial infarction, randomized and open trials were performed with a portable electrocardiographic system coupled with a cellular phone programmed to automatically transmit ECGs to the base hospital. Consecutive patients served by the 6 units of the Salt Lake City Emergency Rescue System were studied; 71 patients were randomized to in-field ECG (n = 34) versus no ECG (n = 37). Time on scene was 16.4 +/- 9.7 minutes for the ECG group versus 16.1 +/- 7.0 minutes for the non ECG group (difference not significant). Time of transport averaged 18.2 +/- 9.9 and 17.6 +/- 13.1 minutes, respectively (difference not significant). Six of 34 patients with in-field ECG showed acute myocardial infarction, qualified for and received thrombolytic therapy at 48 +/- 12 minutes after hospital arrival (range 30 to 60) compared with 103 +/- 44 minutes (p less than 0.01) for 51 historical control patients and 68 +/- 29 minutes for 6 concurrent control patients without in-field ECG. Thus, in-field ECG causes negligible delays in paramedic time, leads to significant decreases in time to in-hospital thrombolysis and may make in-field therapy feasible. In-field ECG may be an important addition to reperfusion strategies.