Fluid therapy is fundamental to the acute resuscitation of critically ill patients. In general, however, early and appropriate goal-directed fluid therapy contributes to a degree of fluid overload in most if not all patients. Recent data imply that a threshold may exist beyond which, after acute resuscitation, additional fluid therapy may cause harm. In patients with acute kidney injury and/or oliguria, a positive fluid balance is almost universal. Few studies have examined the impact of fluid balance on clinical outcomes in critically ill adults with acute kidney injury. Payen and coworkers, in a secondary analysis of the SOAP (Sepsis Occurrence in Acutely Ill Patients) study, now present evidence that there is an independent association between mortality and positive fluid balance in a cohort of critically ill patients with acute kidney injury. In this commentary, we discuss these findings within the context of prior literature and propose that assessment of fluid balance should be considered as a potentially valuable biomarker of critical illness.