Study objective: To identify and determine the rates of delivery and performance of telephone CPR in noncardiac arrest incidents. Design: We studied prospectively all out-of-hospital cardiac arrest and potential cardiac arrest incidents from July 1 through October 31, 1992. Setting: King County, Washington, excluding the city of Seattle. Participants: Persons with cardiac arrest or an initial complaint resembling cardiac arrest who received emergency medical services. Interventions: Dispatcher-assisted telephone CPR. Measurements and main results: Three hundred fifty-eight incidents of cardiac arrest, respiratory arrest, and potential cardiac arrest were reviewed. Telephone CPR was offered appropriately in 61 of 87 cases (70%) and inappropriately in eight of 154 potential cardiac arrests (5.2%) (95% confidence interval, 1.7%, 8.7%). Ventilation instructions were performed appropriately in 52 of 87 cases (60%) and inappropriately in three of 154 potential cardiac arrests (1.9%) (95% confidence interval, 0%, 4.1%). Chest compressions were performed appropriately in 26 of 68 cardiac arrests (38.2%) and inappropriately in two of 173 potential cardiac arrests (1.2%) (95% confidence interval, 0%, 2.8%). Conclusion: We found a low rate of performance of telephone CPR in King County for incidents resembling cardiac arrest. This finding suggests that the protocols designed for dispatcher-assisted telephone CPR effectively screen out those incidents that may initially resemble cardiac arrest.