Chronic Obstructive Pulmonary Disease (COPD) is a disease that progresses through inflammation and the main reason for morbidity in COPD is this inflammation. The rate of appearance of OS (Overlap Syndrome) and COPD together is %1 in the general public. Systemic inflammation increases the severity even more when the two diseases appear together. This is why the morbidity and mortality rates are higher with patients diagnosed with OS. Purpose: The purpose of this study is to find out whether there is a significant difference between the cardiopulmonary exercise test results for patients with the moderate and serious form of COPD. Method: 18 patients were subjected to Cardiopulmonary Exercise Tests (KPET) within the scope of the study. 12 of the patients were diagnosed with COPD and 6 of them with OS. A symptom-limited incremental KPET was performed using a Vmax Encore, USA device. The study was designed and carried out in accordance with principles of the Declaration of Helsinki and with approval from the Ethics Review Board of Uludag University (15th June 2010, 2010-13/2). Informed consent was obtained from all subjects. Findings: The results of the KPET revealed that the maximum oxygen consumption (VO2max ml/kg/min) in the group of patients with COPD was 15,5 ±6,4, and 12,4±6 in the group with OS. There was no significant difference in that sense (P>0,05). There was also no significant difference in carbon dioxide outturn between the two groups (VCO2 L/min) (p>0,05). The maximum work-watt in the group of patients with COPD was 59,2±29, and 45,8±19 in the group with OS. There also was no significant difference between these results (p>0,05). The maximum throb of the group of patients diagnosed with COPD was 120±21 , and 116±18 in the group with OS. The comparison of Oxygen pulse value (ml/Pulse) and maximum throb did not show any significant difference either (p>0,05). The maximum ventilation rate (VEmax-L/min) in the group of patients with COPD was 43±13, and 36,6±9,2 in the group with OS. No significant difference was found between the two groups. (p>0,05). End-tidal CO2 values and end-tidal O2 values did not show any significant difference either. When compared (p >0,05) . The respiratory quotient RQ in the group with COPD was 1,1 ±0,4 whereas in the group with OS the value was 1,1±0,4. No significant difference. The anaerobic tree-shot (AT) values in both groups were similar. No statistically significant difference was found between these two values either. (p>0,05). Conclusion: Although the KPET values of patients diagnosed with COPD and OSD were below the general population, there were no significant differences between the two diseases. Keywords: COPD, Overlap Syndrome, KPET