Study objective: To determine the effect of the addition of adenosine, as a standing-order medication, on the prehospital management of supraventricular tachycardia (SVT) in a large urban emergency medical services (EMS) system. Design: Prospective observational case series with historical controls. Setting: Large urban municipal EMS system staffed by paramedics and emergency medical technicians trained to operate automatic or semiautomatic defibrillators (EMT-Ds). Participants: We observed a consecutive sample of prehospital patients who presented with an initial ECG rhythm of SVT, as interpreted by the treating paramedics, between July 1 and December 31, 1993. We used patients from the same 6-month period in 1992 as our control group. Indications for treatment were chest pain, evidence of myocardial ischemia, or shock. Adenosine had been introduced as a first-line medication to be used under standing orders in cases of unstable SVT before a physician was contacted for medical control options. Results: We studied 239 cases and 228 controls. Acceptable call reports with pretreatment and posttreatment ECGs were available for 140 (59%) of the study cases and 104 (46%) of the controls. The two groups were similar in terms of age, sex, and initial vital signs. In the control group, 75 patients had indications for treatment, and 16 were treated (21%). In the study group, 127 had indications for treatment and 103 (81.1%) were treated (odds ratio, 15.83; 95% confidence interval, 7.38-34.4). Conclusion: The introduction of adenosine as a standing-order medication into an urban EMS system increased the proportion of patients who received advanced life support treatment. Paramedics were able to accurately diagnose and begin treatment of SVT with adenosine without direct medical supervision. [Lozano M Jr, McIntosh BA, Giordano LM: Effect of adenosine on the management of supraventricular tachycardia by urban paramedics. Ann Emerg Med December 1995;26:691-696.]