Abstract： Objective To investigate the clinical characteristics of extremely low birth weight infants (ELBW) and very low birth weight infants (VLBW) with different degrees of eosinophilia,and analyze the correlation between eosinophilia and bronchopulmonary dysplasia (BPD).Method The clinical data of 147 ELBW and VLBW infants admitted to the Department of Pediatrics,the Fifth Medical Center of Chinese PLA General Hospital from January 2019 to December 2022 were retrospectively analyzed.The infants were divided into normal group (eosinophils<0.7×109/L,n=43),mild elevation group[eosinophils was (0.7-0.9)×109/L,n=39]and moderate to severe elevation group (eosinophils ≥1.0×109/L,n=65) according to the degree of elevation in eosinophils.Perinatal data and incidence of postnatal related diseases were compared among all groups.According to whether the infants had BPD and the severity of BPD,they were divided into non-BPD group (n=81),mild BPD group (n=46) and moderate to severe BPD group (n=20),and the differences in eosinophils among the three groups were compared.Statistical methods performed by t-test,analysis of variance,rank sum test,x2 test,Fisher exact probability method,univariate and multivariate Logistic regression analysis and receiver operating characteristic (ROC) curve.Result The incidence of eosinophilia was 70.7% (104/147),with a mild elevation of 26.5% (39/147),a moderate to severe elevation of 44.2% (65/147).The gestational age of moderate and severe elevation group and mild elevation group was lower than that in the normal group,and the differences were statistically significant (all P<0.05).The incidence of septicemia,respiratory distress syndrome (RDS) above degree Ⅱ,BPD and blood transfusion in mild elevation group and moderate to severe elevation group were significantly higher than those in the normal group.The incidence of BPD in the moderate to severe elevation group was higher than that in the mild elevation group.The application time of non-invasive ventilator in mild elevation group and moderate to severe elevation group was longer than that in normal group,and the differences were statistically significant (all P<0.05).There were significant differences in gestational age,septicemia and incidence of eosinophilia,days of ventilator application and blood transfusion between non-BPD group and BPD group (all P<0.05).Multivariate Logistic regression analysis showed that gestational age,eosinophilia,days of ventilator application,blood transfusion and septicemia were independently correlated with the occurrence of BPD (all P<0.05).The area under the curve (AUC) of eosinophils at the third week after birth was the largest (0.772),and the sensitivity and specificity were both higher (70.1% and 76.2%),indicating a good predictive value for BPD.The eosinophils in the mild BPD group was higher than that in the non-BPD group at the third and fourth week after birth (Z=-3.178,-2.475;P=0.001,0.013).The eosinophils in the moderate to severe BPD group were higher than those in the non-BPD group at the third and fourth week after birth (Z=-2.601,-2.460;P=0.009,0.014).The eosinophils in moderate to severe BPD group were higher than those in mild BPD group at the third week after birth (Z=-2.422,P=0.015).Conclusion The incidence of eosinophilia in ELBW and VLBW infants is higher,and eosinophilia is related to the occurrence of BPD.The level of eosinophils at the third week after birth has the highest sensitivity and specificity in predicting BPD,which should be paid attention to clinically.