Objectives To investigate the changes of cystatin C in the serum of patients with acute kidney injury ( AKI) or end-stage renal disease ( ESRD), and study its significance in the early diagnosis of AKI and its correlation with the degree of renal function injury. Method The cases in Xiangya hospital were enrolled in this study according to the RIFLE criteria, including 20 cases of slight acute kidney injury, 30 cases of medium-severe acute kidney injury, 48 cases of victims of the 5. 12 wenchuan earthquake, 32 cases of end-stage renal disease and 20 healthy patients. The microparticle-enhanced immunoturbidimetry method was used to detect serum cystatin C, and the colorimetric method was used to detect urine N-acetyl-beta-D-glucosaminidase (NAGase). The enzymic method was used to detect serum creatinine. The correlation between serum cystatin C and serum creatinine was analyzed, and the sensitivity and specificity of serum cystatin C were evaluated with the receiver operator characteristic curve (ROC curve). Results Compared with healthy control group, the serum cystatin C increased obviously in acute kidney injury group and ESRD group( P <0. 05 and P <0. 01). The serum cystatin C was positively correlated with serum creati-nine( P <0.01). The serum cystatin C in the 5. 12 wenchuan earthquake injured group was also higher than that in healthy control group ( P <0.05), an the serum cystatin C had an AUC - ROC of 0.931 ( P <0. 01). Conclusion Compared to the conventional biomarkers, the earlier emergence of serum cystatin C can contribute better to early clinical diagnosis of AKI. The serum cystatin C is positively correlated with renal function, and reflect changes in renal function accurately.