In 2015, the Los Angeles Fire Department (LAFD) replaced the Medical Priority Dispatch System® (MPDS) with a new, homegrown Los Angeles Tiered Dispatch System (LA-TDS). The objective of this study was to assess the impact of this new dispatch system on telecommunicator-assisted CPR (T-CPR).This was an interrupted time series study of out-of-hospital cardiac arrest (OHCA) cases where resuscitation was attempted with LAFD 9-1-1 telecommunicators using either MPDS® (January 1, 2014 to March 31, 2014) or LA-TDS (January 1, 2015 to March 31, 2015). Trained non-LAFD abstractors listened to all 9-1-1 calls and recorded if T-CPR was initiated, and the elapsed time from the start of the call until key events. The primary outcome was prevalence of T-CPR.Of 1027 calls during the study period, 597 9-1-1 calls met study inclusion/exclusion criteria (including 289 in MPDS cohort, 308 in LA-TDS cohort). The prevalence of T-CPR was significantly greater using LA-TDS (57%) vs. MPDS (43%) (OR 1.86, 95% CI 1.3-2.6). The LA-TDS cohort demonstrated a significant decrease (p 0.001) in time to recognition of cardiac arrest, time to dispatch, and time to first T-CPR chest compression while there was no significant difference in the elapsed time to first description of agonal breathing. For cases where the telecommunicator had the opportunity to assess consciousness and breathing, there was a significant improvement in cardiac arrest recognition in 1 min, prevalence of T-CPR (p 0.001), and T-CPR started in2 min (p 0.001).Implementation of the new Los Angeles Tiered Dispatch System was associated with decreased time to recognition of cardiac arrest and an increased rate of T-CPR compared to the previously used Medical Priority Dispatch System®.