To elucidate the cause of power reduction in endoscopic carbon dioxide (CO2) laser surgery and to define the actual effective power on tissue of a beam delivered through different endoscopes, we designed an experimental model. In passing a CO2 laser beam through a CO2 gas-pressurized chamber, no significant power loss was noted. A correlation between the internal diameter of an endoscope and the power output became evident. Effective power of 92%, 85%, and 48% of the potential 100% was measured when a beam was delivered through single- and double-puncture laparoscopes and a hysteroscope, respectively. These figures are relevant in predicting CO2 laser effect used endoscopically.