Beside measurement of functional residual capacity (FRC) in ventilated children is impractical. Using a simple technique based on open circuit N2 washout, we measured FRC in ventilated children. The system was evaluated in the laboratory and in patients. Using a mechanical lung, the reproducibility of 200 studies over a range of 100-500 mL at each of four different flow rates (10 determinations at each level) was very high with a mean coefficient of variation of 2.3% (range 0.5-5.1%). Linearity of the integrated N2 signal for volumes of 100-500 mL washed out at different flow rates was excellent (range 7.4-17.9 L/min), r = 0.99. The mean difference between measured and preset mechanical lung volumes was 2.4% (range 0-4.6%). In vivo, reproducibility of six to 10 FRC determinations in each of 30 children gave a mean coefficient of variation of 2.7%. Comparison to the conventional Douglas bag collection method showed a high correlation (r = 0.97). We conclude that this is an easy, highly reproducible, and accurate method for FRC determination suitable to ventilated infants and children.