Objective: The implementation of cardiopulmonary resuscitation guidelines, updated every five years, appears to improve patient survival rates after Out-Of-Hospital Cardiac Arrest (OHCA). The aim of this study is: 1) to measure the level of improvement in the prognosis of OHCA patient survival rates for the years 2009 and 2010 and the following two years 2011 and 2012; and 2) correlate the improvement in prognosis with the updated 2010 Advanced Cardiovascular Life Support (ACLS) Guidelines. Method: We performed a retrospective observational study based on Geneva's OHCA register that includes data from January 1, 2009 to December 31, 2012. We compared the evolution of prognostic factors that influenced survival at hospital discharge between the periods before and after the implementation of the 2010 guidelines. We then compared the survival rates between each period. Finally, we adjusted the effects on survival in the second period to prognostic factors not linked with the care provided by Emergency Medical Services (EMS) teams, using a multivariable logistic regression model. Changes in advanced resuscitation treatment provided by EMS personnel were also examined. Results: 795 OHCA were resuscitated between 1st January, 2009 and 31st December, 2012. The prognosis of patient survival at the time of hospital discharge rose from 10.33% in 2009-2010 to 17.01% in 2011-2012 (p = 0.007). After making adjustments for the effect of improved survival rates on the second period with factors not related to care provided by EMS teams, the odds ratio (OR) remains comparable (OR = 1.87, 95% CI [1.08-3.22]). Measured changes in treatment provided by EMS personnel were minor. Conclusions: Survival rate for OHCA patients improved significantly in 2011-2012. This study suggests that it was probably the improvement in the quality of care provided during CPR and post-cardiac arrest care that have contributed to the increase in survival rates at the time of hospital discharge.