Objective: The neutrophil/lymphocyte ratio (NLR) has been evaluated as a new predictor of cardiovascular risk. Inflammation has been shown to be associated with various arrhythmias including supraventricular tachycardias (SVTs). In this study, we aimed to investigate the relation between NLR and SVT in patients with a documented atrial tachyarrhythmia.Methods: The study used a retrospective cross-sectional design. Patients who had SVT but were otherwise healthy were included. The exclusion criteria included drug use (except antiarrhythmic agents), morbid obesity, acute or chronic infection, inflammatory diseases, systemic diseases, and cancer. Total and differential leukocyte counts and routine biochemical tests were performed before the ablation procedure.Results: The study included 150 patients with SVT and 98 healthy controls. The biochemical and hematological parameters were comparable between the groups, except neutrophil and lymphocyte counts. The neutrophil count was significantly higher (4.7±1.5x103/µL versus 4.1±1.0x103/µL; p<0.001) and lymphocyte count was significantly lower (2.2±0.6x103/µL versus 2.5±0.6x103/µL; p=0.001) in the SVT group than in the control group. As a result, the SVT group had significantly higher NLR values than the control group (2.2±0.9 versus 1.7±0.5; p<0.001). In addition, NLR values were higher in patients in whom tachycardia was induced during an electrophysiological study (EPS) (2.3±0.9 versus 2.0±0.8; p=0.02). The association between NLR and SVT remained significant after multivariate analysis (odds ratio: 1.5, 95% confidence interval: 1.001-2.263, p=0.049).Conclusion: Our study indicated that NLR values were significantly higher in patients with documented SVT than in control subjects. Inducibility of SVT during EPS was associated with higher NLR values