Elevated plasma levels of the endogenous nitric oxide ( NO) synthase inhibitor asymmetric dimethylarginine ( ADMA) contribute to endothelial dysfunction and seem to be a predictor for cardiovascular mortality. Elevated ADMA plasma concentrations have been demonstrated in patients with hypertension. However, the plasma concentrations of ADMA in white coat hypertension (WCH) has not been previously studied. The aim of this study was to evaluate ADMA in WCH and compare with normotensive (NT) and hypertensive ( HT) patients. We also evaluated the relation between ADMA and NO in these three groups. For this purpose, 34 NT, 34 white coat hypertensive ( clinical hypertension and ambulatory daytime blood pressure <135/85mmHg) and 34 HT patients were recruited in this study. The subjects were matched for age, gender, body mass index (BMI) and the patients with smoking habit, dyslipidaemia and diabetes mellitus were excluded. The ADMA levels were determined by high performance liquid chromatography. Plasma ADMA levels were significantly higher in WCH group than in the NT group (3.21 +/- 0.49 mu mol/l vs 2.84 +/- 0.58 mu mol/ l, P = 0.046). It was significantly higher in the HT group than in the NTs (4.24 +/- 0.38 mu mol/l, P<0.001). There was also a significant difference between the HT and WCH groups (P<0.001). The WCH subjects had significantly higher levels of NO than the HTs (41.68 +/- 2.23 vs 32.18 +/- 2.68 mu mol/l; P<0.001) and significantly lower values than the NTs (48.24 +/- 4.29 mu mol/l; P<0.001). In WCH and HT group, there was a negative correlation between ADMA and NO ( r = - 0.515, P = 0.003 and r = - 0.389, P = 0.034, respectively). In NT subjects, there was no correlation between these two parameters ( r = - 0.287, P = 0.124). The correlation between ADMA and NO was stronger in WCH group than in HT group. Although NO levels in HT patients were lower than WCHs and ADMA levels were higher in HT patients than WCHs, the negative correlation of these two parameters were more pronounced in WCH group. Decreased NO and increased ADMA levels in WCH may indicate endothelial dysfunction. Our data indicate also that WCH represent an intermediate group between NT and HT when endothelial dysfunction is concerned.