In infants and very young children, functional residual capacity (FRC) is the lung volume most frequently measured and gas dilution techniques are most frequently used to measure FRC. We compared measurements of FRC by helium dilution (FRCHe) and nitrogen washout (FRCN2) in a lung model of known volumes (20, 40, 60, 80 mL) in 8 normal infants (NL), 8 infants with acute respiratory illness (ARI), and 7 infants with chronic lung disease (CLD). In the model lung, measurements of FRCHe had a significantly greater coefficient of variation compared to FRCN2 (6.5 vs 1.5%, P less than 0.02), but there was no such difference in the results from all infants combined (6.5 vs 6.2%). In the model lung, the difference between known volumes and the measured values was significantly different from zero only for FRCHe at 20 mL (-4.4 mL, P less than 0.02). For both FRCHe and FRCN2, the slopes of the regression equations were 0.99 and the intercepts not significantly different from zero. We conclude that techniques for measuring FRCHe and FRCN2 yield accurate, reproducible, and comparable results in normal, healthy infants and very young children, and in those with respiratory disease.