Background <p>Recently, endoscopic thyroid surgeries have gained popularity for the treatment of thyroid tumors. Mivacurium chloride is a highly selective non-depolarizing neuromuscular blocking agent with a short duration of action. Due to this advantage, mivacurium chloride is used for endoscopic thyroid surgeries requiring neuromonitoring. However, mivacurium chloride-related histamine release is a side-effect to be considered in these procedures. There have been no prior case reports documenting mivacurium chloride-associated tissue edema interfering with endoscopic surgery.</p> Case presentation <p>Here we report two cases of clearly discernable tissue edema and fluid exudation during endoscopic thyroidectomy associated with mivacurium chloride administration. Although they received mivacurium chloride doses less than recommended, their blood pressure decreased &gt; 30% after induction and during anesthesia. In both cases, the thyroid gland and surrounding tissue demonstrated overt edema with continuous fluid exudation during the procedure, thereby affecting the surgeon’s field of vision.</p> Conclusions <p>Due to anatomical limitations and limited space in the neck, anesthesiologists should be cautious in using mivacurium chloride during endoscopic thyroid surgery because it may contribute to overt edema that obscures the operative field. Vigilance in the selection and use of neuromuscular blocking agents during endoscopic thyroid surgery is of paramount importance.</p>