Background: Although a high concentration of plasma asymmetric dimethylarginine (ADMA) have been shown to be associated with risk factors of atherosclerosis and may raise the risk of acute coronary syndrome (ACS), the plasma ADMA concentrations in ACS patients and their changes during the course of medical treatment have not been reported yet. Methods: Using high-performance liquid chromatography, we measured plasma ADMA concentrations in newly diagnosed patients with ACS (n = 48), and traced the changes in those concentrations during short-term medical treatment. Results: Age matched 48 normal healthy subjects were recruited for comparison. Patients with ACS had elevated plasma concentrations of ADMA compared with controls (3.13 짹 0.85 vs. 1.57 짹 0.85 mol/L, P < 0.0001). Follow-up measurements of ADMA were available from 31 patients, showing dramatic decreases in ADMA concentrations over two weeks of medical treatment (from 3.27 짹 0.87 to 1.52 짹 0.47 mol/L, P < 0.0001). The plasma ADMA concentrations showed a significant positive correlation with serum C-reactive protein (CRP), and a significant negative correlation with serum high-density lipoprotein (HDL) and also with plasma alpha-tocopherol concentrations at baseline. During treatment, the changes in plasma ADMA concentrations were significantly correlated with changes in total cholesterol/HDL ratios and serum CRP concentrations. Conclusion: The plasma concentrations of ADMA were significantly elevated in patients with ACS and rapidly decreased over two weeks of medical treatment.