The amount of drug delivered from commercially available inhalation devices which reaches the lungs of preschool children is generally low. We therefore studied the efficiency of lung delivery from an optimised combination of delivery device and drug formulation based on individual patient-related factors. In six three-year-old children we compared the delivery of a radiolabelled budesonide solution with a MMD of 4.2 mm from a conventional nebuliser, with that of a radiolabelled budesonide solution with a MMD of 2.5 mm from a perforated vibrating membrane nebuliser. Lung deposition of budesonide delivered from the perforated vibrating membrane nebuliser was 36% and 38% and notably higher than from a conventional nebuliser (maximum 8%). The development of complementary combinations of delivery devices and drug formulations to meet the needs of efficient inhalation therapy in preschool children seems to be a good way of improving the efficacy of inhaled therapy in this age group.