Background: In obstructive uropathy, despite a severe increase in the serum creatinine (Cr) levels, only a mild cystatin C (CysC) increase was previously reported. Therefore, we aimed to determine the availability of serum Cr/CysC ratio in predicting postrenal acute kidney injury (AKI). Materials and Methods: This was a cross-sectional study involving 61-adult patients with heterogeneous AKI cases. Patients with bilateral pelvicalyceal dilatation in renal sonography were considered as postrenal AKI group (n = 15) and others were intrinsic AKI group (n = 46). Venous blood sampling for blood urea nitrogen, Cr and CysC measurements were performed on admission. Results: The mean age of study population was 66.3 +/- 15.5 years; 38 (62%) of which were male. Two groups were similar regarding age, gender, and comorbidities. Cr/CysC ratio was significantly higher in postrenal AKI group (6.9 +/- 3.1 vs. 4.4 +/- 2.1, P = 0.007). Conclusion: We suggest that serum Cr/CysC ratio seems to be a useful diagnostic tool for detection of postrenal AKI cases, especially for the cases without definite hydronephrosis.