Guidelines for use of aerosolised drugs in children are inconsistent. In a study of 14 infants, 22 children, and 4 adults inspired nebulised aerosols were diluted more for large than for small subjects, because of air entrainment which occurred when inspiratory flow exceeded nebuliser flow. Infants under 6 months of age did not entrain air and would receive undiluted aerosols. All other subjects entrained air, which caused up to a 5-fold dilution in inspired aerosol concentration as subject size increased. In subjects who entrained air, the ratio of inspired nebuliser output versus total nebuliser output was relatively constant, and was related to the respiratory pattern. For a given nebuliser solution concentration, infants who do not entrain will inspire more concentrated aerosols than older children. Once entrainment occurs, the mass of drug inspired is largely independent of size. Regimens for nebulised drug delivery in children may require revision.