Background: The study aimed at investigating the role of the eosinophils in the pathogenesis of Bronchopulmonary Dysplasia (BPD).Methods: Thirteen preterm infants with Respiratory Distress Syndrome (RDS), 15 with BPD and 16 healthy were investigated. Venous blood samples were collected from all infants and serum was analyzed for the total amount of eosinophils, neutrophils, levels of ECP and CD9.Results: The eosinophil count was significantly increased in infants with BPD compared to RDS and healthy infants (1414, 797 and 471 PBE/microliter respectively, p=0.03). Furthermore, significantly increased levels of ECP (p=0,002), decreased EG2 and lower levels of CD9 (p=0,01) point at activation of eosinophils. These differences cannot be explained by the differences in GA and BW between the three groups of infants. Moreover, the ECP levels at four weeks of age, were positively correlated with the duration of supplemental oxygen. The eosinophil count fell promptly when steroid treatment was started in the BPD group.Conclusions: Our findings support the concept of an eosinophil contribution to the pathogenesis of BPD in infants.