To evaluate the ability of clinicians involved in the provision of paediatric intensive care to estimate cardiac index in ventilated children, based on physical examination and clinical and bedside laboratory data. Clinicians were exposed to all available haemodynamic and laboratory data for each patient, allowed to make a physical examination, and asked to first categorize cardiac index as high, high to normal, low to normal, or low, and then to quantify this further with a numerical estimate. Cardiac index was measured simultaneously by femoral artery thermodilution (coefficient of variation 5.37%). One hundred and twelve estimates were made by 27 clinicians on 36 patients (median age 34.5 months). Measured cardiac index ranged from 1.39 to 6.84 1/min/m2. Overall, there was poor correlation categorically (kappa statistic 0.09, weighted kappa 0.169) and numerically (r = 0.24, 95% confidence interval 0.06 to 0.41), although some variation was seen among the various levels of seniority. Assuming that objective measurement, and hence manipulation, of haemodynamic variables may improve outcome, these findings support the need for a safe, accurate, and repeatable technique for measurement of cardiac index in children who are critically ill.