Background : Inappropriate fluid therapy during surgery is associated with significant morbidity and mortality. Fewstudies have examined the effects of particular types of fluids (crystalloid or colloid solutions) in surgical patients,especially with the goal of hemodynamic optimization. Isotonic saline is the most commonly used fluid worldwidebut may be associated with potential nephrotoxicity. Hydroxyethyl starch (HES) solutions are widely used in surgicalpatients as a component of goal-directed fluid optimization strategies, but several large multicenter studies havesuggested increased rates of acute kidney injury and adverse events with the use of HES in ICU patients. Despitewhat may be inferred from physiological studies, the benefit and harm of 0.9 % saline and HES during hemodynamictherapy have not been clearly established in surgical patients. Methods/Design : The FLASH trial is an investigator-initiated, prospective, multicenter, randomized, double-blinded,two-arm trial, randomizing 826 patients with moderate-to-high risk of postoperative complications to receive 6 % HES130/0.4 or 0.9 % saline during individualized goal-directed fluid optimization. The primary outcome measure is acomposite of death or major postoperative complications within 14 days following surgery.Thesamplesizewillallowthedetectionofa10%absolutebetween-groupdifferenceintheprimaryoutcomemeasurewith a type 1 error rate of 5 % and power of 95 %, assuming a 5% mortality rate and 20 % morbidity (thus 25 % for thecomposite endpoint). Discussion : The FLASH trial may provide important data on the efficacy and safety of commonly used fluid solutions andcould have a significant impact on future treatment of surgical patients. Trial registration : ClinicalTrials.gov Identifier: NCT02502773. Registered 16 June 2015.