Vascular endothelial growth factor (VEGF) is a potent endothelial cell growth and permeability factor highly expressed in rodent alveolar epithelium after injury and repair. To investigate VEGF synthesis in human lung epithelial cells, we examined VEGF expression by cultured cells under basal conditions and after cytokine treatment or oxidative stress. Basal VEGF expression was detected in transformed human epithelial cell lines (A549 and 1HAEo-) and in primary human bronchial epithelial cells with RT-PCR, Western blot, and immunocytochemistry. Among the cytokines tested, only transforming growth factor-beta1 increased the levels of excreted VEGF(165) as measured by ELISA. Under hypoxia (0% O(2) for 24 h), the VEGF(165) level increased fivefold, and this effect was O(2) concentration dependent. VEGF concentrations in the medium of all the cell types studied reached values similar to those found in bronchoalveolar lavage fluids from normal patients. Endothelial cells (human umbilical vein endothelial cells) exposed to conditioned medium from primary bronchial epithelial cell cultures showed an increased growth rate, which was inhibited in the presence of a specific neutralizing antibody to VEGF. These results suggest that lung epithelial cells participate in the endothelial repair and angiogenesis that follow lung injury through the synthesis of VEGF.