The measurement of respiratory mechanics is frequently performed on intubated patients. This study shows the effect which the turbulent and inertial characteristics of an endotracheal tube have on the accuracy of the measurement. Two techniques are examined utilizing a mechanical model: the least-squares and the phasor technique. It is concluded that both techniques will give erroneous results if the characteristics of the tube are not accounted for. The error is markedly reduced if the turbulent and inertial terms are accounted for in the analysis.