Background: Massively transfused patients have a high mortality rate. Some studies have suggested that transfusion of high ratios of fresh frozen plasma (FFP) to red blood cells (RBCs) may improve resuscitation and mortality. To date there are no studies examining the impact of the FFP to RBC transfusion ratio on mortality in massively transfused cardiac surgery patients . Methods: A single center retrospective cohort study was performed using data from an eight and a half year period (2006 to 2014). Massive transfusion was defined as receiving at least 8 RBC units during surgery. Patients were classified as having received a high FFP to RBC ratio (greater than 1 to 1), a moderate ratio (between 1 to 1 and 1 to 2), or low ratio (less than 1 to 2). 30-day survival was compared between transfusion groups using Kaplan-Meier analysis and Cox-Proportional Hazards modeling was used to identify variables associated with 30-day mortality. The odds of in-hospital mortality and postoperative morbidities were also compared between transfusion groups using unadjusted and adjusted regression analyses. Results: Of 7,492 cardiac surgery patients, 452 (6%) were massively transfused. 30-day mortality was 25.4% and in-hospital mortality was 30.6%. Patients who received a high FFP to RBC transfusion ratio had improved unadjusted 30-day survival (p=0.05). Patients who received either a high or moderate transfusion ratio had improved adjusted 30-day survival when compared to those who received a low ratio (HR for death=0.339, p=0.002 and 0.511, p=0.007). Patients who received a high transfusion ratio were also more likely to survive to discharge, had fewer reoperations for bleeding, and less renal failure compared to those who received a low ratio. High transfusion ratios were also associated with prolonged ventilation and atrial fibrillation compared to low ratios. Conclusions: A high FFP to RBC transfusion ratio may improve survival in cardiac surgery patients with massive intraoperative transfusion, but may increase the risk for prolonged ventilation and atrial fibrillation. Future studies are needed to determine optimal transfusion practices.