Objective: To investigate reliable biomarkers for predicting histological chorioamnionitis (HCA). Design: In this retrospective study, PPROM mothers were treated with a course of dexamethasone. Blood tests for serum indictors were conducted continuously after first injection. Setting: One Shanghai hospital Population: PPROM mothers before 34 0/7 weeks of gestation Methods: Data were compared by General Linear Model analysis. The diagnostic accuracy in predicting HCA were evaluated with ROC curve. Main Outcome Measures: The response of infectious indictors to dexamethasone treatment; the diagnostic accuracy for CRP and PCT in predicting HCA. Results： We found 98 HCA women (62.42%) and 59 CON women (37.58%) in 157 PPROM mothers. WBC and neutrophil significantly increased 24 hours after the first injection in both groups, followed by a decrease at 72 hours post first injection (P<0.05). No significances were found between two groups in WBC, neutrophil and lymphocyte. Both CRP and PCT were significantly higher in the HCA group after first injection. PCT had both high specificity and sensitivity, especially at the baseline (cutoff, 0.031 ng/ml). Furthermore, the positive predictive values (PPV) of PCT were respectively 0.946 and 0.960 at 48 (cutoff, 0.049 ng/ml) and 72 (cutoff, 0.051 ng/ml) hours, which were better than the corresponding PPV of CRP. Conclusion: The response of WBC, neutrophil and lymphocyte to dexamethasone could be differentiated from uterine infection; PCT could be a reliable biomarker for early diagnosis of HCA. Funding: Shanghai Municipal Commission of Health and Family Planning (GWIV-26, 202040128); Pudong Commission of Health and Family Planning (PW2019D-13).