Objectives To determine the incidence, the constitution of causes and the prognosis of acute kidney injury (AKI) among hospitalized patients in a tertiary hospital, and to evaluate the impact of AKI on hospital cost and length of stay (LOS). Methods Patients who were admitted in Zhongshan Hospital, Fudan University from September 1st 2004 to August 31st 2005 were involved. After checking the computer-based data on kidney function, patients with acute kidney injury were picked out and further history reviews were demanded to get the information of clinical characteristics, prognosis, severity of kidney injury and the causes of AKI. Results There were 37 365 admissions during the study period and 1263 met with the diagnostic criteria of acute kidney injury(AKI). The overall incidence of AKI was 3.38%. In-hospital mortality was 1.52% in all discharges while 18.57% in patients with AKI. The multivariable-adjust OR for death associated with AKI was 10.08 (P<0.01). The results of logistic regression analysis suggested that old age and the elevation percentage of serum creatinine were the risk factors of death among AKI patients. Conclusions Acute kidney injury is prevalent in hospitalized patients. A slight elevation of serum creatinine is associated with significantly increased mortality, LOS and hospital cost. Moreover, outcomes are related directly to the severity of acute kidney injury, which characterized by elevation percentage of serum creatinine