BACKGROUND: Inhaled corticosteroids (ICS) constitute the cornerstone of treatment for asthma. Many studies have reported beneficial short term effects of these drugs, but there are few data on the long term effects of ICS on the decline in forced expiratory volume in 1 second (FEV(1)). This study was undertaken to determine whether adults with asthma treated with ICS have a less pronounced decline in FEV(1) than those not treated with ICS. METHODS: Two hundred and thirty four asthmatic individuals from a longitudinal epidemiological study of the general population of Copenhagen, Denmark were divided into two groups; 44 were treated with ICS and 190 were not treated with ICS. The annual decline in FEV(1) was measured over a 10 year follow up period. RESULTS: The decline in FEV(1) in the 44 patients receiving ICS was 25 ml/year compared with 51 ml/year in the 190 patients not receiving this treatment (p<0.001). The linear regression model with ICS as the variable of interest and sex, smoking, and wheezing as covariates showed that treatment with ICS was associated with a less steep decline in FEV(1) of 18 ml/year (p = 0.01). Adjustment for additional variables including age, socioeconomic status, body mass index, mucus hypersecretion, and use of other asthma medications did not change these results. CONCLUSIONS: Treatment with ICS is associated with a significantly reduced decline in ventilatory function.