OBJECTIVE: The aim of the present study was to evaluate a simplified method for FRC measurement. DESIGN: Accuracy and precision of the method were assessed in a physical lung model; reproducibility was tested in 10 mechanically ventilated patients. In each patient FRC was measured at three PEEP levels. SETTING: Post-operative intensive care unit in a university hospital. MEASUREMENTS AND RESULTS: Gas flow, CO2 concentration, and O2 concentration were measured during in- and expiration by pneumotachography, a mainstream capnometer and a sidestream O2-analyser. For FRC-measurement inspiratory O2 concentration was changed by 30%. FRC was determined as mean value of a N2 washout and N2 washin procedure. Evaluation of this method in a lung model shows a good correlation between FRC set in the lung model and FRC measured (FRC measured = 1.028*FRG model + 22.92 ml; r2 = 0.957; n = 30). The mean difference was 4.4% of FRC-reference (range -8.4% to +21.7%). Duplicate determinations in 10 mechanically ventilated patients differed by an average of -2.7% (range -30.1% to +27.3%). CONCLUSION: Our results suggest that the proposed method can be used in daily clinical work.