BACKGROUND: High-quality cardiopulmonary resuscitation (CPR) is associated with improved survival from out-of-hospital cardiac arrest and includes chest compression depth, chest compression rate, and chest compression fraction within international guideline recommendations. Previous studies have demonstrated divergent results of real-time feedback on CPR performance and patient outcomes. This study investigated the association between emergency medical service CPR quality and real-time CPR feedback for out-of-hospital cardiac arrest. METHODS AND RESULTS: This study collected out-of-hospital cardiac arrest data within the Capital Region of Denmark and compared CPR quality delivered by ambulance personnel. Data were collected in 2 consecutive phases from October 2018 to February 2020. Median chest compression depth was 6.0 cm (no feedback) and 5.9 cm (real-time feedback) (P=0.852). Corresponding proportion of guideline-compliant chest compressions for depth was 16.6% and 28.7%, respectively (P<0.001). Median chest compression rate per minute was 111 and 109 (P<0.001), respectively. Corresponding guideline adherence proportion for compression rate was 65.4% compared with 80.4% (P<0.001), respectively. Chest compression fraction was 78.9% compared with 81.9% (P<0.001), respectively. The combination of guideline-compliant chest compression depth and chest compression rate simultaneously was 8.5% (no feedback) versus 18.8% (feedback) (P<0.001). Improvements were not significant for return of spontaneous circulation (odds ratio [OR], 1.08 [95% CI, 0.84–1.39]), sustained return of spontaneous circulation (OR, 1.00 [95% CI, 0.77–1.31]), or survival to hospital discharge (OR, 0.91 [95% CI, 0.64–1.30]). CONCLUSIONS: Real-time feedback was associated with improved guideline compliance for chest compression depth, rate, and fraction but not return of spontaneous circulation, sustained return of spontaneous circulation, or survival to hospital discharge.