BACKGROUND Measuring non-invasive exhaled biomarkers of inflammation may be important in monitoring asthma therapy. OBJECTIVE Evaluate exhaled nitric oxide with add-on leukotriene synthesis inhibitor in moderate-severe persistent asthmatics on combination controllers. METHODS In a non-randomized, non-placebo, single-blind, fixed sequence, pilot study, we evaluated 22 non-smoking, stable, moderate-severe adult asthmatics on maintenance inhaled fluticasone 250 microg/salmeterol 50 microg (F/S) via MDI bid> or =1 yr, with add-on oral zileuton 600 mg qid. Exhaled fractional nitric oxide (FENO) gas exchange, large airway NO, small airway/alveolar NO concentration (CANO), Juniper score and lung function were measured. Asthmatics were studied at baseline only on F/S bid (visit 1), on F/S bid pre and 2 h post first dose zileuton 600 mg (visit 2), and post 4 weeks (visit 3) F/S bid plus zileuton 600 mg qid. Values were compared at each visit and to healthy non-smoking age matched healthy controls with normal lung function. RESULTS Three asthmatics stopped zileuton prematurely (headache and/or nausea) and 19 (12F) age 55+/-17 yr (mean+/-SD) completed the 4-week study. Baseline forced expiratory lung volume in 1 sec (FEV(1)) was 1.6+/-0.7L (53+/-19% pred) (mean+/-SD), FEV(1) over FVC ratio was 64+/-11% and post 180 microg albuterol FEV(1) was 1.8+/-0.7L (56+/-21% pred), and FEV(1) over FVC ratio was 67+/-12%. Baseline Juniper scores were mild (10+/-10) and similar (p=ns) at all visits. Baseline FENO@50 mL/s was 48+/-27 ppb (mean+/-SD), and FENO@100 mL/s was 29+/-16ppb, and were similar (p=ns) at all visits. Large airway NO flux was 2.0+/-1.3 nL/s (52% asthmatics abnormal) and small airway/alveolar NO was 8.0+/-4.0 ppb (79% asthmatics abnormal) and were similar (p=ns) at all visits. Compared to baseline, post 26+/-6 days Zileuton, mean FEV(1) (L)% predicted increased 3.3% predicted (p=0.03), and FEV(1) over FVC ratio increased 2.2% (p=0.03). CONCLUSION In stable, moderate-severe persistent adult asthmatics, large airway NO flux, small airway/alveolar CANO, and Juniper airway scores, were not significantly different on F/S bid vs F/S bid plus Zileuton for 4 weeks, despite significant small increase in FEV(1) over FVC ratio and FEV(1)% predicted.