The study considered consecutive calls by bystanders to the 911 system for patients in cardiac arrest. Patients were initially eligible if the dispatcher determined that they were unconscious and not breathing normally and that bystander CPR was not under way. If the caller was willing to undertake CPR with the dispatcher's assistance, a randomization envelope containing CPR instructions was opened. Dispatchers attempted to exclude patients with arrest due to trauma, drowning, or asphyxiation (from choking, strangulation, or suffocation), as well as patients who were under 18 years of age; and those who had do-not-resuscitate status or were already receiving CPR.