In order to explore the molecular degradation forms of plasma fibronectin (Fn) in patients with chronic obstructive pulmonary disease (COPD) and its clinical value. We measured the concentration of plasma Fn and analysed Fn molecular forms. The results suggested that the increase of Fn degradation is one of the causes of plasma Fn decline in patients with COPD. The obvious correlations were found between the extent of Fn degradation and the severity of the disease. In view of the documented influences of Fn fragments on the host defense, the increase of Fn fragments in plasma of COPD patients may be an additional factor of inflammatory amplification loops and more contributing to the decrease of plasma opsonic activity.