In recent years a number of commercial instruments for on-line gas exchange measurements has been introduced. One of them, the Engström Metabolic Computer (EMC), is here clinically validated as compared to the standard Douglas bag method for gas sampling and a Centronic Mass Spectrometer (CMS) for gas analysis. VO2, VCO2 and RQ were simultaneously measured and calculated with both methods. Twenty individual gas exchange determinations were made at different times on 12 critically ill patients at the intensive care unit. There was a small but significant difference (P less than 0.025) of 4.3% +/- 8.4% (s.d.) between the two methods when they were used to measure VO2 (the EMC giving the lower value). The corresponding value of VCO2 was 2.4% +/- 9.1% (s.d.), and for RQ the difference was -1.98% +/- 7.1% (s.d.). These differences are not significant at the 95% level of significance. Determinations of oxygen uptake during ventilator treatment with standard methods usually involve technical difficulties and are associated with errors of method around 10%. Our results indicate a probable error of about 8.5% for VO2 in clinical situations. Considering the difficulties involved in determinations of gas exchange, the EMC method seems to be a valuable technique with an accuracy within acceptable limits.