BACKGROUND: Emergency medical services (EMS) personnel often are not permitted to honor requests to withhold resuscitation at the end of life, particularly if there is no written do-not-resuscitate (DNR) order. OBJECTIVE: To determine whether EMS personnel from agencies implementing new guidelines would be more likely to withhold resuscitation from persons having out-of-hospital cardiac arrests than would personnel from agencies that did not implement the guidelines. DESIGN: Observational study in which 16 of 35 local EMS agencies volunteered to implement new guidelines for withholding resuscitation. SETTING: King County, Washington. PATIENTS: 2770 patients with EMS-attended cardiac arrest. INTERVENTION: New guidelines adopted by participating agencies permitted EMS personnel to withhold resuscitation if the patient had a terminal condition and if the patient, family, or caregivers indicated, in writing or verbally, that no resuscitation was desired. MEASUREMENTS: Proportion of resuscitations withheld in agencies that implemented new guidelines compared with those that did not. RESULTS: Emergency medical services personnel from agencies implementing new guidelines withheld resuscitation in 11.8% of patients (99 of 841 patients) having cardiac arrests, compared with an average of 5.3% (range, 4.2% to 5.9%) of patients (103 of 1929 patients) in 3 historical and contemporary control groups. Honoring verbal requests alone accounted for 53% of withheld resuscitations in the intervention group (52 of 99 patients) compared with an average of 8% (range, 7% to 9%) in the control groups (8 of 103 patients). LIMITATIONS: The study was not a randomized, controlled trial; individual agencies chose whether to implement the guidelines. CONCLUSIONS: Implementation of new guidelines was associated with an increase in the number of resuscitations withheld by EMS personnel. This increase was primarily due to honoring verbal requests.