Background: FEC-D (5-FU, epirubicin, cyclophosphamide - docetaxel) and dd (dose-dense) AC-P (doxorubicin-cyclophosphamide - paclitaxel) are considered equally effective regimens in the adjuvant treatment of breast cancer (BC), although never compared directly in a clinical trial. We compared the outcome of BC patients (pts) treated with FEC-D, dd AC-P and AC-P (q3 weekly). Methods: Retrospective study including all female BC pts diagnosed in 2003-2009 in Ontario, who received 1 of those regimens (ascertained from New Drug Funding Program and physician claims databases). Primary endpoint: overall survival (OS). Secondary endpoints: emergency room (ER) visits/admissions, risk of heart failure (HF). Analysis were conducted for OS and HF using Kaplan-Meier method and adjusted for confounders using proportional hazard models. Multivariable logistic regressions were performed for ER visits/admissions adjusting for confounders. FEC-D and ddAC-P were compared using propensity score (PS) matching. Results: 8,462 pts were identified, 4,710 (55.7%) received FEC-D, 2,065 (24.4%) AC-P and 1,687 (19.9%) dd AC-P. Patient characteristics were imbalanced between the arms. In unadjusted analyses, 5-year OS was 91.7% for FEC-D, 89.0% for dd AC-P and 87.4% for AC-P; after PS matching, it was 92% for FEC-D and 89% for ddAC-P. After adjusting for confounders, risk of death was lower for dd AC-P than AC-P (HR=0.76; 95% CI 0.61-0.96), and higher for dd AC-P than FEC-D (HR=1.24; 0.99-1.54) and for AC-P than FEC-D (HR=1.61; 1.26-2.06). ER visits / admissions during treatment, were higher for FEC-D (respectively, 50.5% and 23.7%), than AC-P (35.2% and 11.6%) and dd AC-P (30.5% and 8.4%); the differences persisted after PS matching (respectively, 48.2% and 23.5% for FEC-D, 31.1% and 8.7% for dd AC-P; p<0.0001). Risk of HF at 5 years for the PS matched groups was 1.37% for FEC-D and 1.49% for dd AC-P (p=0.71). Conclusions: OS was better for FEC-D and dd AC-P when compared with AC-P. This may be attributable to imbalances in patient characteristics. Propensity score matching for dd AC-P and FEC-D showed no difference in survival or HF risk between the arms but pts treated with FEC-D continued to have more ER visits/admissions.