Abstract Objective: The aim of this study was to assess the level of serum leptin hormone in chronic obstructive pulmonary disease patients during acute exacerbation and in stable conditions and also, to determine if these changes correlate with changes in the ventillatory functions. Methods Sixty cases were included in this prospective study (40 COPD patients and 20 age related smokers without symptoms or signs of COPD and within normal pulmonary functions as a control). Patients and control were divided according to their BMI into obese (BMI ≥30) and non-obese (BMI=18.5-25). Subjects were submitted to full history taking, thorough physical examination, plain chest X-ray, complete blood count, erythrocyte sedimentation rate, Liver and kidney functions, Fasting and post prandial blood sugar, Ventillatory functions, and serum Leptin level measurement. Results Serum Leptin level (ng/ml) was significantly higher (P < 0.001) in stable obese COPD (mean ±SD= 23.85±4.47) patients than obese controls (mean ±SD=20.9±2.7) and stable non-obese COPD (mean ±SD=5.63±1.05) and Stable non-obese COPD cases had significantly higher (P < 0.05) serum Leptin level than non-obese controls (mean ±SD= 6.53±1.19). Serum Leptin level was significantly higher (P < 0.001) in obese COPD cases during exacerbation (mean ±SD= 67.59±9.8) than in non-obese COPD cases during exacerbation (mean ±SD=18.14±4.15). Significant positive correlation between serum Leptin and BMI (kg/m²) of different groups (P < 0.01) [obese control (r= 0.945), non-obese control (r= 0.970), obese COPD in exacerbation (r= 0.812), obese COPD in stable state (r= 0.774), non-obese COPD exacerbation (r= 0.876) and non-obese COPD in stable state (r= 0.799)]. Conclusion: Serum leptin hormone level (ng/ml) was significantly higher in obese COPD cases than in controls and non-obese cases and during exacerbation than in stability which indicates that leptin plays a role in the systemic inflammatory process. Serum leptin hormone level positively correlated with BMI (kg/m²).