Copyright © 2019 by Wroclaw Medical University.Background. Nitric oxide synthase (NOS) is present in the brain and cerebral arteries and it enables the synthesis of nitric oxide (NO), which plays a critical role in brain perfusion. Asymmetrical dimethylarginine (ADMA) is an endogenous NOS inhibitor. Objectives. The aim of this study was to evaluate serum ADMA levels, which are an indicator of endothelial dysfunction of the renal functions in patients with acute ischemic stroke, and to determine whether there is a possible correlation between ADMA and NO levels and the L-arginine-to-ADMA ratio. Material and methods. Fifty-two patients (22 male and 30 female; mean age: 75.2 ±10.1 years) with a diagnosis of acute ischemic stroke in the first 24 h post-stroke and 48 healthy individuals (controls; 13 male and 35 female; mean age: 60.1 ± 7.92 years) were included in this study. The risk factors recorded and evaluated were age and gender of the patients, serum lipid levels, serum ADMA levels, nitrate-to-nitrite ratios, l-arginine, l-arginine-to-ADMA ratios, sedimentation rate, C-reactive protein (CRP), urea and creatinine levels, and glomerular filtration ratio (eGFR). Results. The mean serum ADMA level was 0.48 ±0.23 μM for the patients and 0.36 ±0.18 μM for the controls. The mean NO level was 2.78 ±0.59 μM for the patient group and 4.49 ±2.84 μM for the controls. The ADMA levels for the patient group were significantly higher than for the control group (p = 0.011); the NO levels for the patients were significantly lower than for the controls (p < 0.001). The logistic regression method demonstrated that ADMA and NO levels may be independent risk factors for the patient group, and the receiver operating characteristic (ROC) curve analysis showed that both of these variables were discriminative risk factors. Conclusions. An increased serum level of the NOS inhibitor ADMA was found to be a possible independent risk factor for ischemic stroke.