From the Childhood Asthma Management Program cohort, which was randomly assigned to receive budesonide, nedocromil, or placebo for 4-6 years, we determined the prevalence of and factors associated with at least 1% per year loss in postbronchodilator FEV(1)% predicted. Participants who had a significant reduction in postbronchodilator FEV(1)% predicted (SRP), comprised 25.7% of the cohort (n = 990). Using logistic regression, predictors of SRP at baseline were younger age (p = 0.0005), male sex (p < 0.0001), clinic (p = 0.02), and higher postbronchodilator FEV(1)% predicted (p = 0.02). Examination of the SRPs indicated that the effect of baseline lung function was such that the higher the lung function, the less steep the reduction in postbronchodilator FEV(1)% predicted (p < 0.0001). A similar proportion of SRPs was found in each treatment group. Among the SRPs, the rate of reduction in postbronchodilator FEV(1)% predicted was similar in all treatment groups. At a single site where biomarker assessment was performed, SRPs also had more prominent eosinophilic inflammation during the washout period. The course and mechanisms of lung function reduction or slow lung growth velocity in children with asthma must be defined. [ABSTRACT FROM AUTHOR]