To quantify the degree of association, if any, between lung size and airway size in humans, the ratio of a measurement known to be sensitive to airway size (maximal expiratory flow divided by static recoil pressure at 50% of vital capacity) to one sensitive to lung size (vital capacity) was examined. If lung and airway size changed together, this ratio would be the same for large and small lungs, i.e., for persons with large and small vital capacities. If lung and airway size were independent, then, on average, the ratio would vary as (vital capacity)-1. Data for 21 men 20 to 50 yr of age showed that the ratio to decreased approximately as (vital capacity)-4/3. This is consistent with independence of airway diameter but dependence of airway length on lung size. Data for each of 7 females of comparable age fell below the adult male (smaller ratios at a given size), as did data for 5 boys less than 20 yr of age. These results suggest that women and boys have airways that are smaller relative to lung size than are those of men and that these sex differences develop late in the growth period.