Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of the endothelial nitric oxide synthase (eNOs). ADMA is believed to be implicated in angiogenesis because it regulates the nitric oxide biosynthesis; any pathological abnormalities in ADMA play a crucial role in the pathogenesis and progression of atherosclerosis. The AIM of the present study was to determine the reference range for plasma concentration of ADMA in a sample of Bulgarian population. To establish the reference interval of ADMA plasma levels and study the impact of sex and age we recruited 150 healthy subjects of Bulgarian nationality aged between 18 and 65 years. The selection criteria for the reference group were made to comply with the generally approved recommendations of the International Federation of Clinical Chemistry (IFCC). Plasma concentrations of ADMA were determined using ELISA assay (DLD, Diagnostics, Hamburg, Germany). The reference ranges for ADMA, given as 95% of the measured values, were from 0.22 to 0.69 micromol/1. We found no sex-related differences in ADMA concentration (P > 0.05), which obviates the need for separate reference intervals for men and women. Single-factor dispersion analysis found no age dependency ofADMA (P > 0.05, F = 1.038) in the studied reference group in the age range 18-65 which makes unnecessary establishment of reference intervals for lower age ranges in this age group. The reference values for ADMA plasma concentrations calculated according to the type of distribution of results can be used as baseline criteria in clinical laboratory studies and for clinical purposes.