Malnutrition is an important problem in patients with chronic obstructive pulmonary disease (COPD). It still remains unclear whether malnutrition contributes to poor pulmonary function through a loss of respiratory muscle mass, or if advanced disease and hypoxemia are the causes of weight loss and malnutrition in COPD patients. This study was made to examine the effects of malnutrition on pulmonary function tests (PFTs) in COPD patients. With this purpose 35 stable COPD patients were enrolled in this study. According to their body mass indexes, the subjects were divided in two groups (group 1: cachectic and group 2: non-cachectic). All subjects were performed PFTs, serum tumor necrosis factor-alpha (TNF-alpha) levels, resting energy expenditure (REE), nutrition parameters, and arterial blood gas tension. PFTs were impaired to a greater degree in cachectic than non-cachectic patients. Serum TNF-alpha levels and REE were higher in cachectic patients than in non-cachectic patients. Significant correlations were observed among PFTs, REE, and serum TNF-alpha level. Furthermore there was a significant correlation between serum albumin level and PFTs. This study demonstrated that cachexia had a negative effect on PFTs in patients with COPD. Additionally, our study showed that serum protein levels can affect airway function and diffusing capacity of lungs in COPD. Another result of this study was that; increased REE and serum TNF-alpha levels could contribute to weight loss in patients with COPD. Further studies are needed to demonstrate the effect of nutritional supplementation containing essential amino acids on PFTs in these patients.