Whether leukotriene receptor antagonists exhibit adequate anti-inflammatory effects in the treatment of asthma is still a controversial issue. The aim of the present study was to perform a direct comparison of the effects of a 4-week treatment with either montelukast (10 mg, once a day) or low-dose inhaled fluticasone (100 microg b.i.d.) on functional and inflammatory parameters in steroid-naïve patients with moderate asthma. Forty patients (forced expiratory volume in one second (FEV1), 60-80% predicted) were studied in a double-blind, randomised, crossover design. Treatment periods were separated by 3-8 weeks of washout. At the beginning and end of each period, FEV1, airway responsiveness to inhaled methacholine (provocative concentration causing a 20% fall in FEV1 (PC20)), the level of exhaled nitric oxide (NO) and sputum differential cell counts were determined. Only short-acting beta2-agonists were allowed for relief of symptoms. FEV1 increased by 0.50+/-0.07 L (mean+/-SEM) after fluticasone and by 0.37+/-0.07 L after montelukast (p<0.001, each), and PC20 by 1.33+/-0.13 (p<0.001) and 0.15+/-0.17 (NS) doubling doses, respectively. Correspondingly, percentages of sputum eosinophils were reduced by factor 2.7 (p<0.01) and 1.4 (nonsignificant (NS)), and the levels of exhaled NO (at 50 mL x s(-1)) by factor 2.1 (p<0.01) and 1.1 (NS). These data indicate a comparable bronchodilator action of montelukast and fluticasone in patients with moderate asthma, but additional attenuation of airway inflammation by fluticasone as detectable through noninvasive methods.