Evidence suggests that a preoperative single-dose steroid improves lung function and decreases the incidence of postoperative symptoms; however, this has not been sufficiently proved in modified radical mastectomy for cancer. This study aimed to evaluate the efficacy of preoperative single-dose steroid administration for postoperative lung function and postoperative symptoms in women undergoing modified radical mastectomy for breast cancer.In this controlled clinical trial, conducted between June 2014 and October 2018, we examined 81 patients. Patients received a preoperative single dose of 8 mg dexamethasone (n=41; treatment group) or placebo (sterile injectable water; n=40; control group). We obtained data on postoperative nausea and vomiting and pain intensity and performed spirometry 1 h before and 1, 6, 12, and 24 h after surgery. The use of additional analgesic or antiemetic drugs was recorded. We followed up patients 30 days after discharge and recorded any surgical or medical complications.The age distribution and anthropometric variables of the two groups were similar. Almost 50% of the patients in each group also underwent breast reconstruction. In the treatment group, pain intensity was always lower, the incidence of postoperative nausea and vomiting was lower at 6, 12, and 24 h, and additional analgesics or antiemetics were required less frequently (P