Nebulized aerosols are commonly used to deliver drugs for the treatment of respiratory disease in children, but there are inadequate data on the dose of drug depositing in the lungs in this age group, and the effect of age on this dose, We therefore aimed to quantify total and regional deposition of nebulized aerosol in children of widely differing age.Twelve infants (median age 0.8 yrs, range 0.3-1.4 yrs) who were asleep, and eight older children (median age 10.8 yrs, range 6.3-18.0 yrs) with cystic fibrosis were studied. Radiolabelled normal saline aerosol was generated by a Turret nebulizer, with a driving flow of 9 l.min(-1). Ah subjects inhaled aerosol via the nasal route, whilst the older children undertook a second study with inhalation via the oral route, Following aerosol inhalation, planar and single-photon emission computed tomography (SPECT) scans were obtained.For the nasal route, total lung deposition was lower in infants (median 1.3%, range 0.3-1.6%) than in older children (median 2.7%, range 1.6-4.4%). For the older children inhaling via the nasal or oral route, there was no influence of age on lung, upper respiratory tract, or the sum of upper respiratory tract and lung deposition.We conclude that the dose of a nasally inspired aerosol reaching the lungs of infants who are asleep is approximately half that for older children, when the nebulizer is operating at 9 l.min(-1). Age does not affect deposition of nasally or orally inspired aerosols in older children.