A prospective study was conducted to compare simultaneous intrathoracic and intra-abdominal central venous pressures in 10 critically ill, ventilated paediatric intensive care patients. Central venous pressures were measured using the water column technique over a 6 h study period. There was excellent correlation between intrathoracic and intra-abdominal vena caval pressure measurements (r = 0.974, p < 0.001). The difference between paired measurements did not exceed the limits of agreement (+/- 2SD, -2.36 to 4.42 cm H2O). The mean (SD) difference between readings was small (1.03 +/- 1.69 cmH2O) and was within clinically tolerable limits. These data suggest a clinically useful, close relationship between intra-abdominal and conventional intrathoracic central venous pressure measurement in this group of patients.