Environmental factors may affect the lungs of children by retarding growth of ventilatory capacity. To detect retarded growth, maximum expiratory flow-volume (MEFV) curves were recorded in boys and girls aged 8-14 yr observing the normal patterns of growth. The accuracy with which deviations from the normal patterns of growth may be measured was also determined. Expiratory flow rates and timed expiratory volumes were less reproducible in the same child than was forced vital capacity. At all ages, the shape of girls'' MEFV curves was different from that of boys in that girls had greater maximum expiratory flow rates (.ovrhdot.Vmax) when differences in forced vital capacity (FVC) were taken into account. The ratio of .ovrhdot.Vmax 50% FVC to FVC decreased in both sexes as they grew older. The rates of growth of the MEFV curve measurements and estimates of their reproducibility were used to calculate minimum sample sizes required to detect reatrded growth of ventilatory capacity. To detect a change of 10% in the normal growth rate in .ovrhdot.Vmax of 50% FVC between ages 8-14 yr, nearly 2000 children would need to be studied annually. Smaller samples could be used if changes in FVC growth rates were of primary concern.