The Acute Kidney Injury Network (AKIN) group has recently proposed modifications to the risk of renal failure, injury to kidney, failure of kidney function, loss of kidney function, and end-stage renal failure (RIFLE) classification system. The few studies that have compared the two classifications have revealed no substantial differences. This study aimed to compare the AKIN and RIFLE classifications for predicting outcome in critically ill patients. This retrospective study investigated the medical records of 291 critically ill patients who were treated in medical intensive care units of a tertiary care hospital between March 2003 and February 2006. This study compared performance of the RIFLE and AKIN criteria for diagnosing and classifying AKI and for predicting hospital mortality. Overall mortality rate was 60.8% (177/291). Increased mortality was progressive and significant (chi-square for trend; P < 0.001) based on the severity of AKIN and RIFLE classification. Hosmer and Lemeshow goodness-of-fit test results demonstrated good fit in both systems. The AKIN and RIFLE scoring systems displayed good areas under the receiver operating characteristic curves (0.720 + or - 0.030, P = 0.001; 0.738 + or - 0.030, P = 0.001, respectively). Compared with RIFLE criteria, this study indicated that AKIN classification does not improve the sensitivity and ability of outcome prediction in critically ill patients.