BackgroundThe efficacy and safety of mesh in open surgery for groin hernia is uncertain. We aimed to answer this question in a Cochrane review on mesh in open surgery for groin hernia. MethodsTo find relevant randomised controlled trials (RCTs), we searched electronic databases and conference proceedings, and scanned reference lists of retrieved papers. Two reviewers independently screened titles and abstracts, extracted data, and assessed study quality. We compared the efficacy and safety between procedures that used mesh versus no mesh, between different types of mesh for the same technique, and between different techniques that used mesh. Findings68 RCTs with over 10 000 patients were included, of which 26 RCTs compared mesh with no mesh, 14 RCTs compared different types of mesh in the same technique, 26 RCTs compared the different techniques that use mesh, and two were ongoing RCTs. The study quality was generally moderate. Median follow-up was 24 months (IQR 12–36). Open groin hernia repair with mesh resulted in significantly reduced hernia recurrence rates compared with no mesh (24 RCTs reporting on hernia recurrence, n=4621, 1·3% [32/2391] vs 4·8% [107/2230]; relative risk [RR] 0·34, 95% CI 0·23–0·53) with no increase in the risk of persisting pain after procedures. There were no significant differences in the rate of hernia recurrence between heavy-weight and light-weight non-absorbable mesh or in the risk of persisting pain. Non-absorbable mesh had a slightly lower hernia recurrence rate than did partially absorbable mesh (eight RCTs, n=1794, 2·0% [18/883] vs 3·0% [27/911]; RR 0·78, 95% CI 0·41–1·49) but had a significantly higher risk of persisting pain after procedure (seven RCTs, n=1682, 13·7% [114/830] vs 9·5% [81/852]; RR 1·75, 95% CI 1·09–2·84). There was no significant difference between non-absorbable mesh and biological graft in hernia recurrence rate or persisting pain after procedure; however, the results were based on three small scale RCTs (n=70, n=97, n=30). Hernia recurrence rates did not differ between Lichtenstein mesh technique and other mesh techniques including Prolene Hernia System, PerFix Plug, and preperitoneal mesh repair, nor in persisting pain. InterpretationUse of mesh in open hernia repair significantly reduced recurrence rates. Rigorous long-term RCTs are needed to determine the comparative efficacy and cost-effectiveness of biological graft, and the differences in efficacy and safety between different types of procedures that use mesh. FundingHS Highland Research and Development Committee.