We have compared results obtained from two widely used ward-based blood gas analysers, the Radiometer OSM3 and ABL505, with standard laboratory analysers, for haemoglobin, sodium and potassium measurement in critically ill adult patients. During the study, paired samples for analysis were obtained from 81 patients for haemoglobin, 115 patients for sodium and 95 patients for potassium. There was good agreement between the ward and the laboratory analysers in terms of the mean results for each test. However, the limits of agreement for the ward-based analysers were wide, suggesting that their results should be used with caution.