Type III procollagen peptide (PCP) is a byproduct of type III collagen synthesis and a potential marker of collagen secretion. In chronic diffuse interstitial lung diseases, elevated PCP concentrations have been found in serum as well as in bronchoalveolar lavage fluid. It has been proposed that PCP is a marker of early, active stages of fibrosis. As severe fibrosis is a frequent complication in adult respiratory distress syndrome (ARDS), we investigated PCP in patients with ARDS and compared the results with those from patients requiring mechanical ventilation because of heart failure and after neurosurgical and surgical interventions, and those from spontaneously breathing patients, including healthy volunteers and patients with pneumonia, liver cirrhosis, and renal failure. PCP concentrations in patients with ARDS were extremely elevated compared with those in control subjects (p less than 0.001) and correlated positively with FiO2 (r = 0.71, p less than 0.01). These results support the pathophysiologic concept of early fibrogenesis in ARDS. As preventing pulmonary fibrosis in ARDS is essential in improving survival rate, we believe PCP can be a valuable diagnostic tool in ARDS.