We undertook a retrospective clinical study to determine the relationship of the calculated base deficit and cardiac index in post cardiac arrest patients admitted to the intensive care unit. Twenty consecutive admissions to an 8-bed general adult intensive care unit over a 2-yr period mean age 62.9 yr (range 24–86) included 17 males and 3 females with a diagnosis of post-cardio-respiratory arrest were studied. All patients were receiving mechanical ventilation and had an arterial line and a pulmonary artery catheter inserted on admission. Serial arterial blood gases and cardiac output measurements taken were reviewed for the first 24 h of admission from the case notes. The number of measurements recorded ranged from four to nine arterial gas samples over the 24 h. The cardiac output was measured using thermodilution with 10 ml of cold saline. All the data were collected from measurements taken within 20 min of each other. Linear regression analysis demonstrated no consistent relationship between the calculated base deficit and cardiac index (r = 0.02,P = 0.82) when data were pooled for the study group. In four of the study group however, there appeared to be a good relationship (r = –0.78,r = –0.76,r = –0.82,r = 0.79:P Our data suggest that the calculated base deficit is a poor predictor of the cardiac index in post cardiac arrest patients. We recommend that the base deficit should be applied with caution in the clinical assessment of cardiac output.