Objective: Vitamin B12 deficiency induces hyper-hyperhomocysteinemia by inhibiting intracellular methionine re-methylation. Hyper-hyperhomocysteinemia increases the risk of atherosclerosis. Asymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase and its level elevates in cardiovascular diseases. In this study, we aimed to examine the relationship between asymmetric dimethylarginine and arterial stiffness and atherosclerosis in adolescents with vitamin B12 deficiency. Methods: A total of 88 adolescents with age ranging between 11 and 17 years of age were enrolled for this study. Among them, 50 patients had vitamin B12 deficiency <130 pg/ml and 38 healthy controls had B12 >200 pg/ml. In all cases, the levels of asymmetric dimethylarginine were measured with high performance liquid chromatography method. The carotid artery intima media thickness and left ventricular mass index were measured using echocardiography. All these measurements of the study groups were compared. Results: Both plasma levels of asymmetric dimethylarginine and carotid artery intima media thickness were significantly higher in the vitamin B12 deficiency group than in the control group. Correlation analysis showed significant negative correlation of vitamin B12 with homocysteine, asymmetric dimethylarginine, and carotid artery intima media thickness (p<0.05). Conclusion: Our results suggest that endothelial dysfunction starts in the early stage of adolescent vitamin B12 deficiency, and vitamin B12-deficient adolescents have increased circulating asymmetric dimethylarginine, showing that endothelial dysfunction and increased carotid artery intima media thickness be related to atherosclerosis.