The balance of oxidant and antioxidant status plays an important role in the left ventricular diastolic dysfunction (LVDD) in patients with hypertension (HT). Thiol is an important part of antioxidant system in the body. The aim of this study was to investigate the relationship between plasma thiol levels and LVDD in patients with HT. A total of 138 patients with newly diagnosed essential hypertensive and 20 age-gender matched subjects as control group enrolled in the study. After echocardiographic assessment, the hypertensive patients were divided into three groups: Group 1: without LVDD (n = 41); group 2: with LVDD grade 1 (n = 57); and group 3: with LVDD grade 2 (n = 40). Plasma thiol, lipid and glucose levels were measured in all subjects. Plasma thiol levels were significantly different between the groups (all of p < 0.05). While the lowest thiol level was in the group 3, the highest level was in the control group. The presence of LVDD was correlated with age, systolic and diastolic blood pressure, thiol levels, and history of coronary artery disease and hyperlipidemia (all of p < 0.05). Age and thiol were however independent predictors of LVDD in multivariate analyses (beta = 0.318, p < 0.001, and beta = -0.314, p < 0.001, respectively). ROC-curve analysis revealed that thiol levels over 163 mu mol/L predict LVDD in hypertensive patients with 75% sensitivity and 70% specificity (AUC = 0.783; 95% CI: 0.714-0.852). Plasma thiol is an independent predictor for the presence of LVDD. This suggests that thiol plays a role in the pathogenesis of diastolic function. Increased thiol levels may provide protection against the development of diastolic dysfunction.