The Acute Dialysis Quality Initiative (ADQI) Group published a consensus definition (the RIFLE criteria) for acute renal failure. We sought to assess the ability of the RIFLE criteria to predict mortality in critically ill Thai patients with acute kidney injury (AKI). We performed a retrospective cohort study, in Siriraj Hospital (a large single tertiary care academic center in Thailand) on 121 patients admitted during November 2005-November 2006. We classified patients according to the maximum RIFLE class (class R, class I or class F) reached during their hospital stay. Demographic data, hospital mortality, hospital length of stay, and need of renal replacement therapy was collected. Patients with maximum RIFLE class R, class I and class F had hospital mortality rates of 35.7%, 35.7% and 65.9%, respectively, compared with 20% for patients without acute kidney injury. Overall hospital mortality of the patients in AKI group (Risk, Injury, Failure group) was increased when compared with no AKI group (Odds ratio = 4.2; 95% Confidence Interval, 1.6-10.6; p =0.003). Mortality was not significantly different among those with the "Risk" and "Injury" class of RIFLE AKI compared with those without AKI, but mortality increased significantly with the "Failure" class (Odds ratio = 7.7; 95% Confidence Interval, 2.7-21.8; p < 0.001). There was the highest rate of renal replacement therapy in the failure group (52.3%) compared with no AKI group (5.7%), and injury group (7.1%) (p < 0.001). Acute kidney injury 'risk, injury, failure', as defined by the newly developed RIFLE classification, is associated with increased hospital mortality and renal replacement therapy in critically ill Thai patients.