OBJECTIVE: We sought to evaluate the accuracy of emergency medical services (EMS) activation of the cardiac catheterization laboratory (CCL) for patients with ST-elevation myocardial infarction (STEMI) and its impact on treatment intervals from dispatch to reperfusion. METHODS: We conducted a before-and-after cohort study of patients presenting via EMS with prehospital electrocardiogram findings consistent with STEMI. Before August 20, 2007, percutaneous coronary intervention was initiated after patient arrival. Afterward, EMS providers could activate the CCL if the prehospital electrocardiogram automated interpretation indicated STEMI. All interval times from EMS dispatch to percutaneous coronary intervention were measured via synchronized timepieces. RESULTS: A total of 53 patients, 14 before and 39 after prehospital activation, were included. Emergency medical services CCL activation was 79.6% sensitive (95% confidence interval [CI], 65.2%-89.3%) and 99.7% specific (95% CI, 99.1%-99.9%). Mean door-to-hospital electrocardiogram and mean CCL-to-reperfusion times were unaffected by the intervention. Prehospital activation of the CCL significantly improved mean door-to-balloon (D2B) time by 18.2 minutes (95% CI, 7.69-28.71 minutes; P = .0029) and door-to-CCL by 14.8 minutes (95% CI, 6.20-23.39 minutes; P = .0024). Improvements in D2B were independent of presentation during peak hours (F ratio = 17.02, P < .0001). There were significant time savings reflected in all EMS intervals: 20.7 minutes (95% CI, 9.1-32.3 minutes; P = .0015) in mean dispatch-to-reperfusion time, 22.2 minutes (95% CI, 11.45-32.95 minutes; P = .0003) in mean first medical contact-to-reperfusion time, and 20 minutes (95% CI, 10.95-29.05 minutes; P = .0001) in recognition-to-reperfusion time. CONCLUSIONS: Emergency medical service providers can appropriately activate the CCL for patients with STEMI before emergency department arrival, significantly reducing mean D2B time. Significant reduction is demonstrated throughout EMS intervals.