Magnesium is an essential metal that has important roles in physiological function of the body organs. Ethambutol is an oral antitubercular agent with chelating effects owing to its chemical structure. The aim of present study is to determine whether ethambutol usage can alter serum magnesium concentration in patients with pulmonary tuberculosis. Sixty patients with diagnosis of pulmonary tuberculosis were enrolled in the study. Blood samples were obtained before treatment from patients. Ten days after starting anti tuberculosis therapy, second blood samples were obtained. The amounts of serum magnesium were determined in all samples by spectrophotometric method. Statistical analysis showed that serum magnesium concentrations at baseline (0.61±0.08 mmol/l) and at day 10 (0.62±0.11 mmol/l) were not different. It is possible that ethambotol does not affect magnesium concentration in tuberculosis patients, however further studies about the other cationic trace elements are recommended.