Fibronectin, a multifunctional glycoprotein, is found on cell surfaces and in extracellular fluids, and modulates cellular adhesion and vascular permeability. Lung FN produced by alveolar macrophages acts as a chemoattractant for fibroblasts and stimulates their replication. PFN samples were obtained from 39 neonates with RDS (BW 1853 ± 110 gm; GA 32 wk), 6 of whom developed BPD (BW 1160 ± 48 gm; GA 28.8 wk). TLFN and albumin(AL) concentrations were measured in a subgroup of 15 RDS and 4 BPD infants. Control(C) PFN values were obtained from 20 healthy premature infants on days 1-3 of life (BW 1750 ± 110 gm; GA 31.2 wk). Control TLFN and AL were measured in 7 neonates intubated for nonpulmonary indications. PFN (mcg/ml); TLFN (ngFN/mcgAl). Conclusions: PFN and TLFN are significantly decreased in infants with RDS vs. controls. Infants who develop BPD demonstrate a significant increase in TLFN (days 14-30) vs. infants with RDS (days 1-10). Speculation: 1) Low TLFN and PFN early in RDS may contribute to pulmonary capillary leak. 2) High TLFN may foster development of pulmonary fibrosis in BPD.