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Background: Renal resistance index, a predictor of kidney allograft function and patient survival, seems to depend on renal and peripheral vascular compliance and resistance. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase and therefore influences vascular resistance., Study Design: We investigated the relationship between renal resistance index, ADMA, and risk factors for cardiovascular diseases and kidney function in a cross-sectional study., Setting & Participants: 200 stable renal allograft recipients (133 men and 67 women with a mean age of 52.8 years)., Predictors: Serum ADMA concentration, pulse pressure, estimated glomerular filtration rate and recipient age., Outcome: Renal resistance index., Measurements: Renal resistance index measured by color-coded duplex ultrasound, serum ADMA concentration measured by liquid chromatography-tandem mass spectrometry, estimated glomerular filtration rate (Nankivell equation), arterial stiffness measured by digital volume pulse, Framingham and other cardiovascular risk factors, and evaluation of concomitant antihypertensive and immunosuppressive medication., Results: Mean serum ADMA concentration was 0.72 +/- 0.21 (+/-SD) micromol/L and mean renal resistance index was 0.71 +/- 0.07. Multiple stepwise regression analysis showed that recipient age (P < 0.001), pulse pressure (P < 0.001), diabetes (P < 0.01) and ADMA concentration (P < 0.01) were independently associated with resistance index. ADMA concentrations were correlated with estimated glomerular filtration rate (P < 0.01)., Limitations: The cross-sectional nature of this study precludes cause-effect conclusions., Conclusions: In addition to established cardiovascular risk factors, ADMA appears to be a relevant determinant of renal resistance index and allograft function and deserves consideration in prospective outcome trials in renal transplantation.