Measurements of total compliance of the respiratory system (CRS) by the weighted spirometer technique and functional residual capacity (FRC) by helium gas dilution were attempted in 63 healthy children (aged 2 to 7 years). Weighted spirometry was well tolerated in all but six children, and FRC measurements were successful in 42. Both measurements were reproducible and not affected by posture. Good correlations were found between CRS and height (r = 0.73) and age (r = 0.83) as well as between FRC and height (r = 0.83) and age (r = 0.74). No differences were found between boys and girls. CRS also correlated significantly with FRC (r = 0.67). In five children with cystic fibrosis, measurements of CRS, FRC and specific compliance correlated with disease severity. Our data suggest these well-tolerated techniques may provide a useful means to distinguish the effects of disease from those of growth.