BACKGROUND Acute stress response to heart surgery can cause major morbidity in patients. The trace of selenium is useful for preventing heart damage. Although the trend of selenium changes is of high importance for body balance, the usefulness of routine use and effectiveness of this element for patients under open-heart surgery is still unclear. OBJECTIVE This study aimed at assessing serum selenium level before and after open-heart surgery and the relationship between selenium trace and patient outcome. METHODS This prospective analytical study was performed on 100 patients undergoing open-heart surgery using cardiopulmonary bypass (CPB). In all patients, the serum level was measured at a predetermined time (before surgery and one and two days after surgery). Then, the relationship between serum selenium and patient outcome was assessed. RESULTS The mean serum selenium level in patients on admission was 72.90 ± 14.62 μg/L, one day after surgery was 71.20 ± 15.84 μg/L, and two days after surgery was 71.54 ± 15.97 μg/L. Serum selenium levels were inversely and significantly associated with age (r = -0.183; P = 0.034, 013) duration of surgery (r = -221 and P = 0.014), duration of intensive care unit stay (r = 0.204, P = 0.021), and duration of CPB (r = 0.223, P = 013). CONCLUSIONS Based on this study's results, a low level of selenium can affect patients' outcomes after open-heart surgery.