Background: Inadvertent sodium (Na+) flux may occur during renal replacement therapy (RRT) in ICU. The objective of this study was to estimate sodium flux during RRT. Methods: Between September 2011 to December 2012 we studied 60 ICU patients receiving extended daily dialysis (EDD, Fresenius 4008S) or continuous renal replacement technique (CRRT, Aquarius 6.01). CRRT was categorized as dialysis with continuous veno-venous haemofiltration (CVVH) or haemodiafiltration (CVVHDF). Sodium balance was calculated as the difference between affluent and effluent fluid sodium concentration corrected for volume. The duration of study was either the duration of a single EDD session or 24 h of CRRT. Results: Both EDD and CRRT contributed to a positive Na+ flux. Despite similar demographics, CRRT patients had a greater positive sodium flux (p < 0.001). At multivariate analysis, factors [exp(b) (SE), p] which significantly affected sodium flux in each mode of RRT were: (1) EDD (R2 = 0.42): gradient between RRT Na+ and serum Na+ [20.9 (5.8), p < 0.02], and total litres of exchange [1.5 (0.68), p < 0.04]; (2) CVVH (R2 = 0.77): gradient between RRT Na+ and serum Na+ [21.8 (4.7), p < 0.001], dialysis day [-20.9 (9.8), p < 0.05], and total litres of exchange [5.2 (0.96), p < 0.001]; (3) CVVHDF (R2 = 0.73): gradient between RRT Na+ and serum Na+ [23.8 (3.7), p < 0.001], and total fluid removal [-18.5 (3.26), p < 0.001]. Conclusion: RRT may inadvertently contribute to sodium load in critically ill patients and is affected by multiple factors including gradient between RRT Na+ and serum Na+.