Peak expiratory flow rates of 578 Libyan boys and 527 Libyan girls aged 12 to 21 years were determined. The prediction formulae that best describe the data are as follows: Boys: PEFR = 160 X Stature 1.87 Girls: PEFR = 130 X Stature2.44 Statistically significant correlations were found between peak expiratory flow rate (PEFR) and standing height, sitting height, body mass index, body surface area and age in both sexes. Stature (standing height) is marginally better as an index of body size in explaining the variability of the ventilatory capacity. The relationship between PEFR and age in girls appears to correlate with the relationship between PEFR and body mass index. The difference in the slope for girls when compared with that for boys is attributed to the obesity/over weight of the girls.