Serial thermodilution and impedance cardiac output measurements in two critically ill patients are presented. Impedance cardiography failed to detect changes in cardiac output and provided values lower than those derived from thermodilution. Chest X-ray examination and changes in thoracic impedance suggested that these two patients had significantly increased extravascular lung water. This failure of impedance cardiography is attributed to aberrant electrical conduction though the lungs as a result of increased lung fluid that alters the impedance waveform. Although reliable when used in normal subjects, impedance cardiography appears not to provide accurate measurements in critically ill patients.