Objective Carbon monoxide poisoning (COP) is the leading cause of mortality and morbidity due to poisoning worldwide. Because children are affected more quick and severely from COP, they may require a longer treatment period, even if carboxyhemoglobin (CO-Hb) and/or lactate levels return to normal. Therefore, a new marker that predicts the duration of treatment and the final outcomes of COP is needed. Methods This case control study was conducted on 32 carbon monoxide-poisoned patients younger than 18 years who had been admitted to pediatric emergency department. The control group included age- and sex-matched 30 healthy children. Blood samples were obtained for analysis of arterial blood gases, CO-Hb percent, methemoglobine, lactate, and asymmetric dimethylarginine (ADMA). Results Asymmetric dimethylarginine levels were significantly increased (P < 0.05) in patients with COP on admission and after the treatment when compared with controls (1.36 [0.89-6.94], 1.69 [0.76-7.81], 1.21 [0.73-3.18] nmol/L, respectively). There was no positive correlation between CO-Hb and ADMA levels on admission and at 6 hours (P = 0.903, r = 0.218, P = 0.231, r = 0.022, respectively). Positive correlation was found between lactate and CO-Hb levels on admission (P = 0.018, r = 0.423). Conclusions This study showed that ADMA levels were still high after 6 hours of 100% oxygen therapy in children with COP, even CO-Hb and/or lactate levels return to normal range. On the basis of these results, we consider that ADMA may be a useful biomarker in patient with COP.