Objectives: AKI is associated with high mortality in critically ill patients. Thus, a definite diagnosis of AKI can prevent complications. However, a diagnostic method of AKI is lacking. the laboratory derangements related with CKD are also insufficient for the determination of the severity of AKI. We tried to identify the impact of the severity of AKI on serological biomarkers including PTH in AKI setting. Methods: Two hundred-eighteen patients with AKI were enrolled. We defined baseline serum creatinine as the lowest value from recent 1 year and analyzed for recent fiver years. The severity of AKI was also classified according to AKIN stages. Serum PTH was categorized with three groups: mild group, moderate group, high group and very high group.  Also, we measured plasma levels of several markers, such as serum phosphate, calcium, creatinine, alkaline phosphatase, C-reactive protein, haemoglobin, to identify correlation between biological markers in AKI. The relationship between categorized factors including PTH and stages of AKI was analysed and compared. Results: One hundred-fifty patients were classified as AKIN stage 3 and only 21 patients had AKI of AKIN stage 1. Baseline characteristic of total enrolled patients showed as followings: age, 70.06 ± 15.31 years; diabetes, 37.72%; oliguria, 21.82%; rhabdomyolysis, 8.63%; dialysis, 32.72%, respectively. Serum PTH level was not significantly correlated with serum creatinine. However, when AKI patients were categorized with four groups (mild~very high) according to PTH levels, all patients in very high group presented with AKI stage 3 (93.8%). The categorized serum PTH levels impacted on AKIN stages statically significant. (p = 0.007) (Table 1)  Conclusions: This study showed that the categorical serum PTH levels can be a predictable marker for determining the severity of AKI. This study has several limitations. Nevertheless, this study is the first study to identify a significant correlation between serum PTH levels and the severity of AKI