One hundred five consecutive patients with mild or moderate asthma not earlier treated with inhaled corticosteroids and with a need of an inhaled bronchodilator of three or more doses a week, and/or asthma symptoms during day or night, and/or peak expiratory flow (PEF) or FEV1 less than 75% of predicted normal values were given an inhaled corticosteroid for 2 years (budesonide delivered via an inspiratory flow-driven multidose dry powder inhaler [Turbuhaler]). According to duration of symptoms, they were divided into six groups; from a duration less than 6 months up to a duration more than 10 years. PEF and FEV1 were measured before and after treatment for 3 months, 1 year, and 2 years. In the groups of patients with a duration of symptoms less than 2 years, mean FEV1 and PEF were significantly higher at all time points as compared with the baseline and as compared with the groups of patients with a longer duration of asthma symptoms. The maximum effects were usually seen after 1 year's treatment with maintained control during the second year. A significant negative correlation was found between duration of symptoms and maximum increases in PEF (r = -0.34; p = 0.0006) and FEV1 (r = -0.32; p = 0.0012), a correlation remaining also after correcting for baseline airway function. No correlation was found between the age of the patients or earlier regular use of beta 2-agonists and improvements in airway function. The results give some evidence that early treatment of asthma with an inhaled steroid may prevent patients from developing chronic airway obstruction. They also support current asthma treatment guidelines advocating early introduction of inhaled anti-inflammatory drugs.