Objective. Emerging evidence indicates a relationship between bronchopulmonary dysplasia (BPD) and chorioamnionitis. Recent data provide evidence of an acute thymic involution in very low birth weight (VLBW) preterm infants and fetuses with histologic chorioamnionitis. We tested the hypothesis that a small thymus detected at birth on the routine chest radiograph is a predictor of BPD in VLBW infants. Methods. A prospective study was conducted on 400 VLBW preterm infants who survived >4 weeks (mean gestational age: 27.5 weeks [range: 24–30]; mean birth weight: 1010 g [range: 450-1450]). Thymic size was measured on routine chest radiographs taken in the first 6 hours after birth and expressed as the ratio between the transverse diameter of the cardiothymic image at the level of the carina and that of the thorax (CT/T). The accuracy of CT/T for identifying infants with BPD was tested using receiver operating characteristic curve analyses and multivariate logistic regression. Results. Fifty-one VLBW infants (12.7%) subsequently developed BPD. A small thymus (CT/T <0.28) was observed in 94.1% of the infants with BPD versus 2.9% of the infants without BPD. A small thymus at birth identified infants with BPD with 94.1% sensitivity and 98.3% specificity (odds ratio: 17.8; 95% confidence interval: 5.7–55.4). Conclusions. A small thymus at birth on the standard chest radiograph can accurately identify VLBW infants who subsequently develop BPD.