BACKGROUND: A direct relationship between dialysate-to-plasma sodium gradient, blood pressure and interdialytic weight gain exists in hemodialysis (HD) patients. The aim of this study was to delineate the long-term variability of serum sodium in HD patients. METHODS: We performed a retrospective cohort analysis of serum sodium and other analytes routinely evaluated in 100 stable chronic HD patients observed for 12 months. RESULTS: Individual levels across the cohort varied from 122 to 145 m M , but 12-month intraindividual coefficients of variation for sodium were low (pre-HD = 1.6%; post-HD = 1.8%) with overall variability similar to that related to laboratory assay variability especially when compared with other analytes (3.1-30.8%). Pre-HD serum sodium had a trend toward hyponatremia (mean 136 +/- 0.8 m M ). CONCLUSION: Serum sodium is stable over time in HD patients. Pre-HD serum sodium may be used as a parameter for individualizing dialysate sodium prescription.