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We developed a modified nitrogen washin/washout technique based on standard monitors using inspiratory and end-tidal gas concentration values for functional residual capacity (FRC) measurements in patients with acute respiratory failure (ARF). For validation we used an oxygen-consuming lung model ventilated with an inspiratory oxygen fraction (Fio2) between 0.3 and 1.0. The respiratory quotient of the lung model was varied between 0.7 and 1.0. Measurements were performed changing Fio2 with fractions of 0.1, 0.2, and 0.3. In 28 patients with ARF, duplicate measurements were performed. In the lung model, an Fio2 change of 0.1 resulted in a value of 103 ± 5% of the reference FRC value of the lung model, and the precision was equally good up to an Fio2 of 1.0 with a value of 103 ± 7%. In the patients, duplicate measurements showed a bias of −5 mL with a 95% confidence interval [−38; 29 mL ]. A comparison of a change in Fio2 of 0.1 with 0.3 showed a bias of −9 mL and limits of agreement of [−365; 347 mL]. This study shows good precision of FRC measurements with standard monitors using a change in Fio2 of only 0.1. Measurements can be performed with equal precision up to an Fio2 of 1.0.