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Background: Although prescribed to every patient undergoing surgery, maintenance fluid therapy is a poorly researched part of perioperative fluid therapy. The tonicity of the chosen solutions, could be an important cause of morbidity, with hyponatremia being a potential side effect of hypotonic solutions, where isotonic solution could lead to fluid overload. Methods: The TOPMAST-trial is an ongoing prospective single-center double-blind randomized trial comparing an isotonic and a hypotonic maintenance fluid strategy during and after surgery in patients undergoing different types of major thoracic surgery. Patients receive NaCl 0.9% in glucose 5% with an added 40 mmol L-1 of potassium chloride or a premixed solution containing 54 mmol L-1 sodium, 55 mmol L-1 chloride and 26 mmol of potassium at a rate of 27 mL per kg of body weight per day. The primary hypothesis is that isotonic maintenance solutions cause a more positive perioperative fluid balance than hypotonic fluids. Different secondary safety endpoints will be explored, especially the effect of the study treatments on the occurrence electrolyte disturbances (e.g. hyponatremia, hyperchloremia) and a set of clinical endpoints. Efficacy endpoints include the need for resuscitation fluids and assessment of renal and hormonal adaptive mechanisms. An anticipated 68 patients will be included between March 2017 and January 2018. Discussion: The study will provide the most comprehensive evaluation of clinically important outcomes associated with the choice of perioperative maintenance fluid therapy.