Functional residual capacity (FRC) can be assessed reliably in young children using a helium gas dilution technique. The aim of the present study was to determine if differences in lung volume are found in asthmatic children of different ethnic origins. Eighty-eight children were studied, 53 of Caucasian origin and 35 of Afro-Caribbean origin. Their median age was 6.1 years (range, 5.3-9.0 years). FRC measured by helium gas dilution is reported as a percentage of predicted for height, using a published regression equation as well as regression equations appropriate for the ethnic origin of the child. In the majority of both groups FRC was elevated above the ethnically appropriate regression equation for healthy children. The Caucasian children apparently had higher FRCs than the Afro-Caribbean children if comparing the absolute lung volume, and if lung volume was expressed as percent of predicted for height, by a regression equation uncorrected for ethnic origin (P less than 0.02). This difference disappeared when ethnically appropriate regression equations were used. We conclude that lung volumes are similar in asthmatic children of different ethnic origin and that it is important to use ethnically appropriate regression equations.