. ABSTRACT: Apoptosis and proliferation and the effect of exogenous surfactant on these processes were investigated in the lungs of mechanically ventilated/oxygen-treated preterm infants with respiratory distress syndrome and stillborn foetuses.Apoptotic and proliferation indices were determined in lung tissue sections from 27 ventilated/oxygen-treated preterm infants and 29 stillborn foetuses. The effect of exogenous surfactant on apoptosis and proliferation was studied in 16 ventilated preterm infants; 11 untreated infants served as control. Apoptotic and proliferating cells were identified by double labelling combining terminal deoxynucleotidyltransferasemediated deoxyuridine triphosphate nick end-labelling or Ki-67 with cell marker proteins. Pathways to cell death were explored by immunolabelling of cleaved caspases-3, -8 and -9.In the lungs of ventilated/oxygen-treated preterm infants, the numbers of apoptotic and proliferating cells increased significantly compared to the respective numbers in the lungs of stillborn foetuses. Apoptosis was detected in alveolar epithelial cells, whereas epithelial, endothelial and smooth muscle cells proliferated. Surfactant treatment reduced apoptosis induced by ventilation/oxygen-treatment; however, the decrease was not significant. Caspases-8 and -9 do not contribute to ventilation-induced apoptosis, whereas caspase-3 is involved.In conclusion, ventilation/oxygen-treatment induces epithelial cell apoptosis and proliferation of epithelial, endothelial and smooth muscle cells in the lungs of preterm infants. Apoptosis and proliferation are involved in several processes during embryonal and foetal life and play an important role in normal cell turnover and tissue development.Although they have been implicated as mechanisms of obtaining correct cell number, they are involved in pathological processes. In the lung, these processes can result from oxygen injury caused by high oxygen concentrations. However, oxygen therapy is often necessary for patients suffering from pulmonary diseases. This is particularly true for preterm infants and neonates suffering from respiratory distress syndrome (RDS), who require supraphysiological concentrations of oxygen to maintain adequate blood oxygen tensions.In animal models, it has been demonstrated that apoptosis is significantly increased in lungs exposed to hyperoxia [1][2][3]. In addition, it could be shown that hyperoxia induces increased expression of genes encoding apoptosis-promoting proteins [1,2]. Proliferation, however, ceases as a consequence of hyperoxia in cell culture experiments with alveolar epithelial cells, fibroblasts and tracheal smooth muscle cells [4][5][6].Exogenous surfactant therapy remains an established treatment of RDS. Recently, it has become increasingly obvious that exogenous surfactant preparations have significant effects on cell physiology and inflammatory processes in the lung comprising suppression of pro-inflammatory cytokines, superoxide production and regulation of the pulmonary host defence [7,8]. Ho...