Abstract: In this study we aimed to determine the effect of gender differences on serum asymmetric dimethyl-l-aiginine (ADMA) levels in renal failure patients. Serum ADMA concentrations and other factors were measured in 59 subjects, 25 control subjects (males/females 13/12) and 34 patients with renal failure (males/females 16/18). Patients attended Nephrology Department in King Abdulaziz University Hospital (Jeddah, Saudi Arabia) between December 2009 and February 2010. ADMA concentrations were measured by competitive ELISA. The serum concentrations of ADMA were significantly elevated in patients with renal failure when compared with their matched control subjects in both sexes. In males, the Mean±SD concentration of ADMA was 1.49±0.18 vs. 0.62±0.23 μmol L-1 (p<0.001). In females, ADMA concentrations were 1.31±0.27 vs. 0.59±0.16 μmol L-1 (p<0.01). In patients with renal failure, ADMA concentrations were significantly decreased in females as compared with males (1.31±0.27 vs. 1.49±0.18 μmol L-1, p<0.05). There was no significant difference in serum ADMA concentration in control male and female subjects. Thus, the present available data suggested that accumulation of ADMA showed gender differences in renal failure patients, which probably contribute to a higher risk of cardiovascular diseases and total mortality in males.