There is increasing evidence that fluid overload and acute kidney injury (AKI) are associated but the exact cause-effect relationship remains unclear. Wang and colleagues analysed patients admitted to 30 intensive care units in China and found that fluid accumulation was independently associated with an increased risk of AKI and mortality. This commentary focuses on the close pathophysiological link between AKI and fluid overload and discusses the implications for clinical practice. It outlines some of the challenges, including the difficulty in diagnosing fluid overload reliably with current methods, and stresses the importance of personalised fluid therapy with physiological end-points to avoid the deleterious effects of fluid overload.