The first derivative of the thoracic impedance (dZ/dt) was recorded in 81 patients entering the Cardiac Care Unit with diverse acute cardiovascular illnesses. An abnormal diastolic impedance waveform was identified in 30 of the patients. These were compared with the other 51 patients admitted under similar circumstances who did not show this abnormal waveform. Retrospective analysis of these 81 patients reveals that the ones showing the abnormal wave eventually had a poorer prognosis (66 per cent were eventually classified as Class III and IV) versus 90 per cent of the patients without the abnormality who were eventually classified under Class I and II (X2 = 28.6; p less than .001). More than 50 per cent of the 30 patients who showed the abnormality died, and 16 out of the 18 who died belonged to the group who showed the abnormal waveform (X2 = 26.7; P less than .001). From this analysis it appears that the presence of this abnormal early diastolic waveform of the dZ/dt tracing can be used as a predictor of outcome in patients with several myocardial insults, both in terms of functional disability and death.