Purpose: Risk stratification of patients prior to surgery is important for reduction of postoperative morbidity and mortality. The frailty concept has been put forward as a good predictor of surgical outcomes. Sarcopenia (depletion of muscle mass) can be used to measure frailty. We aimed to systematically review the literature where core muscle size measurements have been used for risk assessment of patients undergoing major abdominal surgery. Methods: PubMed and EMBASE databases were searched for studies that investigated core muscle size measured with abdominal CT scans and outcomes after major abdominal surgery. Results: Eight studies were found. Four studies investigated postoperative complications related to core muscle area. Three of these studies found significantly increased risk of complications related to low core muscle area. Three studies investigated length of hospitalization, and two of these found significantly longer length of stay related to low core muscle area. Seven studies investigated 1-year and long-term mortality after surgery, whereof only one did not find significantly increased mortality related to low core muscle area. Furthermore, one study found increased short-term (<30 days from surgery) mortality related to low core muscle area. Conclusions: Assessment of core muscle size on preoperative CT scans is an easily obtainable, objective, and robust prognostic risk indicator of postoperative complications and mortality.