© 2014 Elsevier Inc. The Resuscitation Outcomes Consortium is conducting a randomized trial comparing survival with hospital discharge after continuous chest compressions without interruption for ventilation versus currently recommended American Heart Association cardiopulmonary resuscitation with interrupted chest compressions in adult patients with out-of-hospital cardiac arrest without obvious trauma or respiratory cause. Emergency medical services perform study cardiopulmonary resuscitation for 3 intervals of manual chest compressions (each ~2 minutes) or until restoration of spontaneous circulation. Patients randomized to the continuous chest compression intervention receive 200 chest compressions with positive pressure ventilations at a rate of 10/min without interruption in compressions. Those randomized to the interrupted chest compression study arm receive chest compressions interrupted for positive pressure ventilations at a compression:ventilation ratio of 30:2. In either group, each interval of compressions is followed by rhythm analysis and defibrillation as required. Insertion of an advanced airway is deferred for the first ≥6 minutes to reduce interruptions in either study arm. The study uses a cluster randomized design with every-6-month crossovers. The primary outcome is survival to hospital discharge. Secondary outcomes are neurologically intact survival and adverse events. A maximum of 23,600 patients (11,800 per group) enrolled during the post-run-in phase of the study will provide ≥90% power to detect a relative change of 16% in the rate of survival to discharge, 8.1% to 9.4% with overall significance level of 0.05. If this trial demonstrates improved survival with either strategy, >3,000 premature deaths from cardiac arrest would be averted annually.