BACKGROUND: With the current practice of using estimated glomerular filtration rate (eGFR) for the assessment of renal function, serum urea is arguably a redundant test. However, with little evidence, it is purported that urea can be used as a marker to aid in the assessment of hydration status. The aim of this study was to compare serum urea and eGFR with urine specific gravity (USG) to establish how each compares with this surrogate marker of hydration status. METHODS: The study subjects comprised 2,547 separate acute hospital attendances (1,489 female, 1,058 male; median age (IQR) 60 (39-81) years) where USG and serum urea and creatinine were measured immediately on admission. RESULTS: A significant rise in the median serum urea concentration was observed with increasing categories of USG (p < 0.0001). In contrast, a significant trend was not observed for eGFR vs. USG (p = 0.65). CONCLUSION: Serum urea concentration is significantly affected by hydration status whereas eGFR is not. Copyright 2009 S. Karger AG, Basel.