Background: An increased prevalence of masked hypertension (MHT) has been demonstrated among patients with diabetes mellitus (DM). MHT appears to cause cardiovascular (CV) complications similar to clinically overt hypertension. Asymmetric dimethylarginine (ADMA) is an endogenous nitric oxide inhibitor and higher plasma levels of ADMA are related to increased CV risk in both the general population and among patients with DM. The aim of this study was to evaluate the relationship between MHT and ADMA in diabetic patients. Methods: This study included DM patients (n = 131) with normal office blood pressure (<140/90 mmHg). None of the participants were using antihypertensive medications. All participants utilized an ambulatory blood pressure monitor (ABPM) for 24 h. Serum ADMA and arginine levels were measured using the fluorescence detector high performance liquid chromatography method. Results: The prevalence of MHT was 24.4% among the study subjects. ADMA levels were increased in the MHT group when compared with normotensive diabetics (6.2 ± 2.2 vs 4.2 ± 1.7 μmol/L p = 0.001, respectively). Furthermore, arginine/ADMA ratio was lower in the MHT group than among the normotensive group (29.9 ± 12.1 vs 46.0 ± 19.0 p = 0.001). In the multivariate logistic regression model, ADMA, BMI and HDL levels were found to be independent predictors of MHT Odds ratio: 1.63 (1.28-2.06), 1.19 (1.05-1.35), and 0.95 (0.90-0.99), respectively. The cut-off value of the ADMA was 4.34 μmol/L with a sensitivity, specificity, positive predictive value, and negative predictive value of 84.4%, 59.6%, of 40.3%, and 92.2%, respectively (AUC = 0.78). Conclusions: Serum ADMA may play a role in both the pathophysiology and screening of MHT in DM subjects. © 2013 Elsevier Ireland Ltd.