Currently, no consistent evidence-based guidelines for the management of synchronous bilateral breast cancer (SBBC) exist and it is uncertain how presenting with SBBC affects patients' prognosis. We conducted a review of studies analyzing the association between SBBC and prognosis. The studies that reported adjusted effect measures were included in meta-analyses of effect of bilaterality on breast cancer mortality. From 57 initially identified records 17 studies from 11 different countries including 8,050 SBBC patients were included. The quality of the studies varied but was generally low with small sample sizes, and lack of consistent, detailed histo-pathological information. When doing meta-analysis on the subgroup of studies that provided adjusted effect estimates on breast cancer mortality (nine studies including 3,631 SBBC cases), we found that bilaterality in itself had a negative impact on prognosis after adjustment for known prognostic factors (pooled HR 1.37, 95 % CI 1.24-1.50, p < 0.0001). Multiple sensitivity analyses indicated robustness of the overall estimate. This review summarizes the current evidence of the association between SBBC and prognosis. The previously accepted convention that appropriate adjuvant treatment can be determined by considering the higher risk cancer was not confirmed in this review; rather it seems that being diagnosed with two tumors simultaneously entails a worse prognosis above and beyond that of the unilateral cancers of the same stage. To determine the true association between SBBC and breast cancer prognosis, studies of large and updated samples of SBBC should be done and include thorough histo-pathologic information.