We report serial measurements of lung volume and airflow in small children after lung transplantation. We expected that immature lungs could grow and develop normal volumes after transplantation, despite denervation and immunosuppression. At predetermined intervals, functional residual capacity (FRC) and forced expiratory flow were measured 86 times in 23 recipients younger than 3 yr of age (age at transplant, 13.2 +/- 8.4 mo; range, 2 to 30 mo). FRC was measured using open-circuit N2 washout. Maximal flow at FRC by rapid thoracoabdominal compression was used to distinguish between infants with and those without airflow obstruction. The slope of FRC (in milliliters) versus body length (in centimeters) for all 23 recipients studied was 8.63. For those children without obstruction (flow at FRC >/= 0.9 FRC/s, n = 16), the slope of FRC versus length was 6.61. The coefficient of variation for FRC measurements for all infants was 3.90 +/- 2.80% (range, 0.3 to 16.9%). We conclude that in the absence of significant airflow obstruction the volume of transplanted immature lungs increases at a rate similar to that reported in normal infants.