OBJECTIVE: To test whether the newly developed comprehensive complication index (CCI) is more sensitive than traditional endpoints for detecting between-group differences in randomized controlled trials (RCTs). BACKGROUND: A major challenge in RCTs is the choice of optimal endpoints to detect treatment effects. Mortality is no longer a sufficient marker in studies, and morbidity is often poorly defined. The CCI, integrating all complications including their severity in a linear scale ranging from 0 (no complication) to 100 (death), is a new tool, which may be more sensitive than other traditional endpoints to detect treatment effects on postoperative morbidity. METHODS: The CCI was tested in 3 published RCTs from European centers evaluating pancreas, esophageal and colon resections. To compare the sensitivity of the CCI with traditional morbidity endpoints, for example, presence of any (yes/no) or only the most severe complications, all postoperative events were assessed, and the CCI calculated. Treatment effects and sample size calculations were compared using the CCI and traditional endpoints. RESULTS: Although RCTs failed to show between-group differences using any or most severe complications, the CCI revealed significant differences between treatment groups in 2 RCTs-after pancreas (P=0.009) and esophageal surgery (P=0.014). The CCI in the RCT on colon resections confirmed the absence of between-group differences (P=0.39). The required sample sizes in trials are up to 9 times lower for the CCI than for traditional morbidity endpoints. CONCLUSIONS: This study demonstrates superiority of the CCI to traditional endpoints. The CCI may serve as an appealing endpoint for future RCTs and may reduce the sample size.