BACKGROUND Symmetric dimethylarginine (SDMA) is a by-product of protein methylation. Once released from proteins, SDMA is eliminated by the kidneys; consequently, plasma concentration has been suggested as a sensitive marker of renal function. Furthermore, recent work implicates SDMA in the pathogenesis of cardiovascular disease. To date, reference limits for SDMA plasma concentrations in healthy individuals are lacking. METHODS This study defined reference limits for plasma SDMA concentrations in 840 relatively healthy individuals of the Offspring Cohort from Framingham Heart Study (mean age 56 years, 61% women). Plasma SDMA concentrations were determined by LC-MS/MS using a stable isotope dilution assay. RESULTS The median SDMA concentration in the reference sample was 0.37 μmol/L (Q1, Q3:0.32, 0.43 μmol/L) and the reference limits were 0.225 and 0.533 (2.5th and 97.5th percentile). In a multivariable regression model, serum creatinine, age and total homocysteine were positively associated with SDMA (p<0.001 for all), whereas the body mass index and diastolic blood pressure were inversely related to SDMA (p-values<0.01 and 0.03, respectively). CONCLUSIONS This study reports plasma SDMA reference limits from the community-based Framingham Heart Study. Plasma SDMA concentration was related positively to advancing age, but inversely to renal function. These reference limits may allow the identification of individuals with raised plasma SDMA concentrations.