Background Chronic obstructive pulmonary disease (COPD) is a consequence of an underlying chronic inflammatory disorder of the airways that is usually progressive and causes dysregulation in the metabolism of collagen. Prolidase has an important role in the recycling of proline for collagen synthesis and cell growth. Objective: We measured and compared prolidase activity in healthy individuals with COPD patients to find out that whether its activity might reflect disturbances of collagen metabolism in the patients. We also investigated oxidative-antioxidative status and its relationship with prolidase activity in this disease. Methods. Thirty voluntary patients with COPD and 30 healthy control subjects with similar age range and sex were included into the study. Plasma prolidase activities, total antioxidant capacity (TAG) and lipid peroxidation (LPO) levels were measured in the patient and control groups. Results: Plasma prolidase activity and TAC levels were significantly lower, and LPO levels were significantly higher in the patients than those in the control subjects (P<0.05, P<0.001, and P<0.001, respectively). Significant correlations were detected between plasma prolidase activity and TAG and LPO levels in the patients group (r=0.679, P<0.001; r= 426, P<0.05, respectively). Conclusions: The results suggest that oxidative-antioxidative balance and collagen turnover are altered by the development of COPD in human lungs, and prolidase activity may reflect disturbances of collagen metabolism in this pulmonary disease. Monitoring of plasma prolidase activity and oxidative-antioxidative balance may be useful in evaluating fibrotic processes and oxidative damage in the chronic inflammatory lung disease in human. J. Clin. Lab. Anal. 25:8-13, 2011. (C) 2011 Wiley-Liss, Inc.