The concentrations of hyaluronan (HA) were measured in bronchoalveolar lavage (BAL) fluid and serum from 12patients with adult respiratory distress syndrome (ARDS). The median BAL fluid HA concentration was 353 llg/L, about six times higher than that seen in control patients (p< 0.001). The median serum HA value wa.. 619 llg/L, which was a 30-fold increase compared with that in the control patients (p< 0.001). Another connective tissue component, type III procollagen peptide, was not recovered in significant amounts during lavage in patients with ARDS, but it appeared in the bloodstream in increased concentrations (p< 0.001). Obtained recovery of HA during lavage of patients with ARDS cannot be explained by an enhanced passive leakage from the bloodstream because of increased alveolar-capillary permeability, but rather could reflect a mobilization of HA from lung interstitial tissue because of hydrostatic mechanisms. Alternatively, the appearance of