Metered-dose inhalers (MDIs) and dry powder inhalers play an important role in the treatment of asthma in children of all ages. Yet these devices, which were originally developed for use in adults, interact differently with children. Through childhood there are progressive changes in pharmacokinetic handling and pharmacodynamic effects of inhaled antiasthmatic drugs, in the efficiency and distribution of aerosolized drugs in the respiratory tract, and in the patient's ability to successfully use aerosol devices. This, in turn, produces changes in potential for producing efficacy and adverse effects, and in the balance between risk and benefit. These differences from adults are greatest for children under 4-5 years of age, who are unable to use DPIs or unassisted MDIs, and who therefore must rely on nebulizers and MDIs with valved holding chambers for inhaled drug delivery. Unfortunately, there are no drugs approved for delivery via MDI (with holding chamber) in children under 4 years of age, and there are insufficient data to ensure that many of the available drug-MDI-holding-chamber combinations are both safe and effective. In particular, the potential for effects of inhaled corticosteroids on growth are insufficiently studied in this age group and remains a concern. It is likely that the risk of adverse effects on growth are different for each of the many possible MDI/valved-holding-chamber combinations.