Seven critically ill children had simultaneous measurement of whole blood lactate concentrations obtained from a systemic arterial and mixed venous (pulmonary artery) site. An excellent correlation was found (r = 0.995). The mean difference between arterial and mixed venous values was 0.02 mmol/l and the limits of agreement (+/- 0.22) were -0.20 to 0.24. The differences found were clinically insignificant (two-tailed paired Student's t-test; p = 0.36) and therefore support the continued use of arterial sampling for blood lactate measurement.