Asthma, a chronic and potentially life-threatening disease of the airways, affects patients of all ages. Inhaled corticosteroids (ICS) are the recommended first-line therapy for patients with persistent asthma. To review the clinical efficacy and tolerability data available on budesonide in the treatment of mild-to-moderate persistent asthma, a MEDLINE database search was performed for 1996-2003 using the following key words: budesonide, inhaled corticosteroid, efficacy, safety, systemic. When administered once or twice daily, budesonide effectively controls asthma in children, adolescents, and adults with mild-to-moderate asthma. Budesonide can be delivered effectively via a dry powder inhaler (Pulmicort Turbuhaler) in patients aged > or = 6 years or as an inhalation suspension (Pulmicort Respules) in children as young as 12 months. With over 20 years' clinical exposure, budesonide has been demonstrated to be well tolerated in the treatment of chronic asthma in patients as young as 12 months. Specifically, at doses required to treat mild or moderate persistent asthma, budesonide does not affect hypothalamic-pituitary-adrenal axis function, bone mineral density, cataract formation, or final adult height. As Pulmicort Turbuhaler, budesonide is the only ICS to achieve a Food and Drug Administration pregnancy category B rating. Early intervention with budesonide is recommended in asthma management: maximum benefit from therapy is reported in patients treated within 2 years of disease recognition. Budesonide is effective and well tolerated in the control of mild-to-moderate persistent asthma in patients aged 12 months and older. There is no evidence for variation in efficacy in population subgroups.