BACKGROUND: Variations exist in the surgical techniques used for caesarean section and many have not been rigorously assessed in randomised controlled trials. We aimed to assess whether any surgical techniques were associated with improved outcomes for women and babies. METHODS: CORONIS was a pragmatic international 2×2×2×2×2 non-regular fractional, factorial, unmasked, randomised controlled trial that examined five elements of the caesarean section technique in intervention pairs. CORONIS was undertaken at 19 sites in Argentina, Chile, Ghana, India, Kenya, Pakistan, and Sudan. Each site was assigned to three of the five intervention pairs: blunt versus sharp abdominal entry; exteriorisation of the uterus for repair versus intra-abdominal repair; single-layer versus double-layer closure of the uterus; closure versus non-closure of the peritoneum (pelvic and parietal); and chromic catgut versus polyglactin-910 for uterine repair. Pregnant women were eligible if they were to undergo their first or second caesarean section through a planned transverse abdominal incision. Women were randomly assigned by a secure web-based number allocation system to one intervention from each of the three assigned pairs. All investigators, surgeons, and participants were unmasked to treatment allocation. The primary outcome was the composite of death, maternal infectious morbidity, further operative procedures, or blood transfusion (>1 unit) up to the 6-week follow-up visit. Women were analysed in the groups into which they were allocated. The CORONIS Trial is registered with Current Controlled Trials: ISRCTN31089967. FINDINGS: Between May 20, 2007, and Dec 31, 2010, 15 935 women were recruited. There were no statistically significant differences within any of the intervention pairs for the primary outcome: blunt versus sharp entry risk ratio 1·03 (95% CI 0·91-1·17), exterior versus intra-abdominal repair 0·96 (0·84-1·08), single-layer versus double-layer closure 0·96 (0·85-1·08), closure versus non-closure 1·06 (0·94-1·20), and chromic catgut versus polyglactin-910 0·90 (0·78-1·04). 144 serious adverse events were reported, of which 26 were possibly related to the intervention. Most of the reported serious adverse events were known complications of surgery or complications of the reasons for the caesarean section. INTERPRETATION: These findings suggest that any of these surgical techniques is acceptable. However, longer-term follow-up is needed to assess whether the absence of evidence of short-term effects will translate into an absence of long-term effects. FUNDING: UK Medical Research Council and WHO.