Measurements of thoracic gas volume (TGV), airway resistance (Raw), and airway conductance (Gaw) were calculated in a group of 42 normal infants using a whole-body plethysmograph. Maximum expiratory flow at functional residual capacity was measured in a separate group of 108 normal infants. Using data obtained from these infants the following regression equations were calculated: Gaw (L.s-1.cmH2O) = -0.0475 + 0.00164 x length (cm) square root of TGV (mL1/2) = -3.22 + 0.263 x length (cm) VmaxFRC (mL.s-1) = -173 + 5.2 x length (cm). The standard errors of prediction are a measure of the scatter of individual results from the normal population about the true regression line. They were used to define limits of the normal ranges for these tests of lung function, and to develop a scoring system. This approach is preferable to expressing results as percent predicted, which gives no indication of how likely a measurement is to be within normal limits.