As can be seen in Figure 2, the present values in infants tend to be somewhat higher than those published previously; this can be because we report the highest attainable maxFRC values, whereas previous publications reported a mean of at least two efforts.18,19 We chose to report the highest value rather than a mean, as all international guidelines on spirometry now advocate, for infants, preschool children, and adults.1,3,15 Another possibility for higher values is that we did not account for the “age-to-height” effect described recently because of the relatively small groups of subjects we examined.7 Our values were similar to those presented by Wall et al,28 who measured maxFRC in a group of preschool children of similar age and height range, who performed voluntary PEFV. Their mean maxFRC value of 1.15 ± 0.29 compares favorably with ours (1.00 ± 0.31 L/s). Buist et al,29 from the same center, measured maxFRC in a group of 73 boys and girls from preschool to school age. They, too, had their subjects performing voluntary PEFV maneuvers. Although their values are somewhat lower than ours, they fall within the 95% CI of our data. It is worth noting that in this age group, our regression is heavily weighted with children performing voluntary spirometry. A small fraction of the group was studied supine and under ketamine anesthesia.20 Our maxFRC values were similar to those of Wall et al28 and Buist et al.29 In that respect, we were also interested to see if our reference equation describing normal values of maxFRC as a function of body length could be applied to older children and young adults. We calculated maxFRC values for older populations from publications25 that presented explicit equations for FVC, FEF50%, FEF75%, and IC, from which maxFRC could be calculated by the algorithm described here and in e-Appendix 1. Based on these correlations, maxFRC was found to be 2.86 L/s and 3.43 L/s for an 18-year-old 170-cm-tall man and woman, respectively. Extrapolating our nonlinear correlation to this height yields a maxFRC value of 3.23 L/s. To the best of our knowledge, Molho et al30 reported the only study in which maxFRC values were directly extracted from routine FVC maneuvers in 85 healthy adults ranging in age from 18 to 78 years. Based on their correlation, the mean maxFRC for an 18-year-old 170-cm-tall man was 3.201 L/s, again showing a very good agreement with our extrapolated values to adulthood. It is, thus, possible that the present correlation of maxFRC to height may be applicable for a much wider age range.