Bariatric surgery is the best long term treatment for morbid obesity. However, it carries risks of considerable morbidity and potential mortality. There is no published review on pre-operative identification of high-risk patients in bariatric surgery. This systematic review analyses obesity surgery mortality risk score (OS-MRS) as a tool for pre-operative prediction of mortality risk in bariatric surgery. Medline and Embase was systematically searched using the medical subjects headings (MeSH) terms 'bariatric surgery' and 'mortality' with further free text search and cross references. Studies that described OS-MRS to predict mortality risk after bariatric surgery were included in this review. Six studies evaluated 9,382 patients to assess the validity of OS-MRS to predict the mortality risk after bariatric surgery. Patient's age ranged from 19 to 67 years, and the body mass index ranged from 30 to 84. There were 83 deaths among the 9,382 patients (0.88 %) with individual studies reporting a mortality range from 0 % to 1.49 %. There were 13 deaths among 4,912 (0.26 %) class A patients, 55 deaths among 4,124 (1.33 %) class B patients and 15 deaths among 346 (4.34 %) class C patients. Mortality in classes A, B and C was significantly different from each of the other two classes (P < 0.05, χ(2)). This systematic review confirms that OS-MRS stratifies the mortality risk in the three-risk classification subgroups of patients. The OS-MRS can be used for pre-operative identification of high-risk patients undergoing primary Roux-en-Y gastric bypass surgery.