During mechanical ventilation high fluctuations of introthoracic pressures occur. No data have been presented as to the effects of positive pressure ventilation in patients with chronic obstructive pulmonary disease on the assessment of cardiac output from thoracic electrical bioimpedance. In order to evaluate a thoracic electrical bioimpedance system for the measurement of cardiac output, 40 paired values of cardiac output in 12 mechanically ventilated patients with chronic obstructive pulmonary disease were measured simultaneously with the standard thermodilution method. The data were plotted as the mean for the two measurement techniques versus the difference. The chosen acceptable difference for limits of agreement (mean difference +-2SD) was set at 22%. The mean difference was 0,1 liter per minute and standard deviation 0,7 liter per minute (12,2%). 92% of differences for two measurement techniques did not exceed the chosen acceptable difference.The two methods ore therefore considered to be in agreement. The continous and noninvasive nature of impedance method and its simplicity of use makes this technology very useful for the titration of drug and fluid therapy. It provides warning from impending problems which might require immediate treatment.