In 2005, the American Heart Association (AHA) released guidelines to improve survival rates from out-of-hospital cardiac arrest (OHCA). Objective. To determine if, and when, emergency medical services (EMS) agencies participating in the Resuscitation Outcomes Consortium (ROC) implemented these guidelines. Methods. We contacted 178 EMS agencies and completed structured telephone interviews with 176 agencies. The survey collected data on specific treatment protocols before and after implementation of the 2005 guidelines as well as the date of implementation crossover (the "crossover date"). The crossover date was then linked to a database describing the size, type, and structure of each agency. Descriptive statistics and regression were used to examine patterns in time to crossover. Results. The 2005 guidelines were implemented by 174 agencies (99%). The number of days from guideline release to implementation was as follows: mean 416 (standard deviation 172), median 415 (range 49-750). There was no difference in time to implementation in fire-based agencies (mean 432), nonfire municipal agencies (mean 365), and private agencies (mean 389, p = 0.31). Agencies not providing transport took longer to implement than agencies that transported patients (463 vs. 384 days, p = 0.004). Agencies providing only basic life support (BLS) care took longer to implement than agencies who provided advanced life support (ALS) care (mean 462 vs. 397 days, p = 0.03). Larger agencies (>10 vehicles) were able to implement the guidelines more quickly than smaller agencies (mean 386 vs. 442 days, p = 0.03). On average, it took 8.9 fewer days to implement the guidelines for every 50% increase in EMS-treated runs/year to which an agency responded. Conclusion. ROC EMS agencies required an average of 416 days to implement the 2005 AHA guidelines for OHCA. Small EMS agencies, BLS-only agencies, and nontransport agencies took longer than large agencies, agencies providing ALS care, and transport agencies, respectively, to implement the guidelines. Causes of delays to guideline implementation and effective methods for rapid EMS knowledge translation deserve investigation. Copyright © 2010 Informa Healthcare USA, Inc.