Previous studies have revealed the relationship between asthma with obesity and low body mass index (BMI) with chronic obstructive pulmonary disease (COPD). Tumor necrosis factor-alpha (TNF-alpha) is thought to be related with low BMI. The aim of this study was to determine sputum and serum TNF-alpha levels in patients with COPD and asthma and to evaluate whether these parameters had correlation with BMI. Thirty patients with moderate persistent asthma and 26 patients with moderate -severe COPD were included. After BMI values were calculated, sputum was induced by inhalation of hypertonic saline solution and blood was drawn for analysis of serum TNF-alpha levels. There were significant differences in age, serum and sputum TNF-alpha levels between asthma and COPD subjects (Sputum TNF-alpha: asthma; 513 +/- 151 pg/mL-COPD: 333 +/- 126 pg/mL, p< 0.001; Serum TNF-alpha: asthma; 332 +/- 114 pg/mL-COPD: 197 +/- 81 pg/mL, p< 0.001), however there was no difference in BMI (asthma; 28 +/- 5.7-COPD; 26.6 +/- 12.9, p= 0.1). Patients were divided into four categories according to their BMI values as underweight, normal, overweight and obese. In asthmatics; there were 12 (40%) obese and 11 (36%) overweight patients while 9 (34%) of COPD patients were underweight. No significant difference was observed among these four groups according to serum-sputum TNF-alpha and smoking history both in asthmatics and in COPD subjects. While there was no correlation between BMI and serum-sputum TNF-alpha levels, BMI was significantly correlated with both smoking history and duration of disease in COPD patients. As a result, most of the asthmatic patients were described as overweight and obese while no such variation was noted in the COPD patients. The induced sputum TNF-alpha levels has no additional benefit on serum TNF-alpha levels which has already known to be associated with BMI.