Measurements of ventilatory capacity have been made in 664 healthy school-children (ages 5-16 years) of European, Afrocaribbean and Indian stock living in the same environment in inner-city Nottingham: 100%, 95% and 80% of these three groups respectively had been born in the UK. The ventilatory capacity data have been analyzed by logarithmic regressions on stature. For forced vital capacity (FVC) and forced expiratory volume (FEV1) a large difference between European and Indian children was found, amounting to 13% at a given stature. The Afrocaribbean children have values similar to those of the Indians. There were no material ethnic differences for peak expiratory flow rate or for FEV1/FVC(%). The differences in FVC and FEV1 appear to be attributable to constitutional rather than environmental influences.