BACKGROUND The fraction of exhaled nitric oxide (FeNO) is a useful marker of asthma control. The FeNO measurement with our offline method using SIEVERS bag collection kit may be more affordable, but there have been no studies to show the effect of anti-asthmatic therapy on FeNO with our offline method. METHODS The study population comprised 36 steroid-naïve asthmatics at our outpatient clinic. We treated them according to asthma prevention and management guideline 2006, Japan. We also measured eNO levels by our offline method and spirometory on baseline, 4weeks, and 12 weeks of treatment. RESULTS All asthmatics were symptom-free on 12 weeks of treatment. The levels of FeNO FEV1/FVC were significantly decreased on 4 weeks and 12 weeks of treatment, compared with that on first visit. We classified the subjects into two groups; (A) FEV1/FVC <70% (n=11) or (B) FEV1/FVC > or =70% (n=25) on baseline. In (A) group, the level of FeNO and FEV1/FVC were significantly improved on 4 and 12 weeks of treatment. In (B) group, on 4 weeks of treatment, the level of FEV1/FVC was significantly increased but the level of FeNO was not significantly changed. On 12 weeks of treatment, the levels of FeNO was significantly decreased, but the level of FEV1/FVC was not significantly changed. CONCLUSION The levels of FeNO were decreased by antiasthmatic therapy, so that offline monitoring of eNO will facilitate the management of bronchial asthma in patients treated with these drugs.