BACKGROUND: Lung deposition of inhaled steroids, likely to be of benefit in the anti-inflammatory treatment of asthma in young children, is low. This is explained by age specific anatomical and physiological characteristics as well as poor cooperation with aerosol therapy. However, total lung deposition and the ratio of lung deposition to oropharyngeal deposition are key determinants of clinical efficacy and of systemic side effects of aerosolized drugs. OBJECTIVES: The aim of this study was to determine lung deposition and ratio of lung deposition to oropharyngeal deposition using a modified vibrating membrane nebuliser to deliver budesonide with a small particle size, taking into account the needs of young children. PATIENTS AND METHODS: Ten asthmatic children (5 males), mean age 20.3 months (range 6-41 months) inhaled radiolabelled budesonide (MMD 2.6microm) through a modified vibrating membrane nebuliser (modified PARI e-Flow). Lung deposition expressed as a percentage of the emitted dose was measured using scintigraphy and the ratio of lung deposition to oropharyngeal deposition was calculated. RESULTS: Mean lung deposition (SD) expressed as percentage of emitted dose and mean lung to oropharyngeal deposition ratio (SD) in quietly breathing children (n=5) and in children crying during inhalation were 48.6% (10.5) versus 20.0% (10.9), and 1.0 (0.3) versus 0.3 (0.2), respectively. CONCLUSIONS: We have shown that by using an improved age-adjusted complementary combination of delivery device and drug formulation to deliver small particles, lung deposition and ratio of lung deposition to oropharyngeal deposition in young asthmatic children is highly improved. But the main factor limiting aerosol delivery in this age group remains cooperation.