The purpose of this study was to evaluate changes in lung function and fractional exhaled nitric oxide (FeNO) in children with mild to moderate persistent asthma treated with low-dose inhaled steroids but still manifesting significantly increased residual volume (RV). This was a crossover study evaluating lung function and FeNO before and after a 2-week crossover therapy period by adding either montelukast or salmeterol to inhaled fluticasone propionate in 12 children with asthma. Salmeterol increased forced expiratory volume at 1 second (FEV(1)) and decreased RV without effects on eNO. Addition of montelukast produced a statistically significant decrease both for RV (from 191.7 +/- 60.8% to 132.4 +/- 36.1%; p = 0.03) and for eNO (from 14.0 +/- 6.3 ppb to 8. 5+/- 5.0 ppb; p < 0.01). No effect was observed for the montelukast treatment period in FEV(1) and forced expiratory flow at 25-75%. Add-on therapy may reduce RV and decrease levels of FeNO, leading to consideration of a possible anti-inflammatory additive effect that could improve the control of the disease.