The determination of functional residual capacity (FRC) would be extremely helpful for the controlled adjustment of mechanical ventilation in sick neonates and infants. However, these patients have small lung volumes and usually have been intubated by uncuffed endotracheal tubes (ETT). Therefore, the open-circuit nitrogen washout technique (N2wo) may give false FRC values if the inspired oxygen concentration (FIO2) is high and leakage around the ETT is present. We evaluated the N2wo as supplied by the Pediatric Pulmonary System 2600 (Sensor-Medics) in a small-sized lung model by 570 measurements using five different ventilator settings, an FIO2 increasing up to 0.9, different bypass flows between 0 and 12 L/min, and various patterns of leakage, either during inspiration or exhalation, or both. We found the most reliable results (error, 0.6%; CV, 0.7%) with a bypass flow of 6 L/min. Absolute N2 volumes as small as 14 mL could be measured using an FIO2 as high as 0.9 with only slight loss of accuracy (error, 4%; CV, 2.8%). During leakage, FRC had been underestimated with a very strong correlation to the total amount of leakage over the measurement period, which was irrespective of the ventilatory parameters (r = 0.9, P < 0.001). The regression equation could, therefore, be used for FRC correction in the lung model. However, most of the miscalculation was due to N2 loss during expiratory leakage, which quite simply and reliably can be excluded by an end-inspiratory occlusion test.(ABSTRACT TRUNCATED AT 250 WORDS)