Intravenous thrombolytic therapy improves left ventricular function and reduces mortality in patients with acute myocardial infarction (AMI). In European and Middle Eastern trials, prehospital delivery of thrombolytic agents by physician-directed mobile intensive care units has been successful. This report describes two independently conceived and performed trials that used cellular telephone transmission of 12-lead ECGs to deliver recombinant tissue plasminogen activator (r-tPA) in the field to patients with AMI. In the Nashville Prehospital TPA Trial, 85 patients with chest pain were evaluated in the field for possible administration of r-tPA over a 6-month period. Three of 85 patients (3.5%) were found to be actual candidates for r-tPA treatment in the field. In phase II (dry-run phase) of the Cincinnati Heart Project, 374 patients were evaluated in the field with 14 documented cases of AMI (3.7%) before r-tPA was placed in ambulances for administration by paramedics. In phase III (active with r-TPA in ambulances), over a 1-year period 103 patients were evaluated with six (5.8%) documented cases of AMI. Three of five r-tPA field treatment decisions by emergency physicians using transmitted 12-lead ECGs were accurate (60%). When patients in phases II and III were combined, only 20 of 477 total patients (4.2%) were documented to have AMI. A decline in paramedic skills was noted because of the infrequent administration of the thrombolytic agent. Combining the Nashville and Cincinnati experiences, only 27 of 562 total patients with chest pain (4.8%) were candidates for prehospital thrombolysis. We conclude that few patients evaluated in the prehospital setting are actual candidates for thrombolytic therapy. Substantial allocation of financial and human resources for prehospital delivery of intravenous thrombolytic therapy does not appear warranted.