Objective It has been suggested that fluid accumulation may delay recognition of acute kidney injury (AKI). We sought to determine the impact of fluid balance on the incidence of non-dialysis requiring AKI in patients with acute lung injury and to describe associated outcomes, including mortality. Design Analysis of the Fluid and Catheter Treatment Trial, a factorial randomized clinical trial of conservative versus liberal fluid management and of management guided by a central venous versus pulmonary artery catheter. Setting and Patients 1000 patients at ARDS Network hospitals. Measurements and Main Results The incidence of AKI, defined as an absolute rise in creatinine of ≥ 0.3 mg/dL or a relative change of > 50% over 48 hours, was examined before and after adjustment of serum creatinine for fluid balance. The incidence of AKI before adjustment for fluid balance was greater in those managed with the conservative fluid protocol (57 versus 51%, p = 0.04). After adjustment for fluid balance, the incidence of AKI was greater in those managed with the liberal fluid protocol (66 versus 58%, p = 0.007). Patients who met AKI criteria after adjustment of creatinine for fluid balance (but not before) had a mortality rate that was significantly greater than those who did not meet AKI criteria both before and after adjustment for fluid balance (31 versus 12%, p < 0.001) and those who had AKI before but not after adjustment for fluid balance (31 versus 11%, p = 0.005). The mortality of those patients meeting AKI criteria after but not before adjustment for fluid balance was similar to patients with AKI both before and after adjustment for fluid balance (31% versus 38%, p = 0.18). Conclusions Fluid management influences serum creatinine and therefore the diagnosis of AKI using creatinine-based definitions. Patients with “unrecognized” AKI that is identified after adjusting for positive fluid balance have high mortality rates, and patients who have AKI before but not after adjusting for fluid balance have low mortality rates. Future studies of AKI should consider potential differences in serum creatinine caused by changes in fluid balance and the impact of these differences on diagnosis and prognosis.