Background: This study compared the effect of inhaled fluticasone propionate (FP) with the combination of salmeterol/fluticasone propionate (SFC) on lung function parameters in patients with mild asthma.Methods: Adult patients with mild persistent asthma (≥ 80% predicted FEV1) receiving 200-500 μg of BDP or equivalent were randomised to receive either FP 100 μg or SFC 50/100 μg twice daily from a Diskus® inhaler for four weeks. The primary outcome was the change from baseline in airway resistance (sRaw) at 12 hrs post dose measured by whole body plethysmography. Impulse oscillometry and spirometry were also performed.Results: A comparison of the geometric mean sRaw at 12 hrs post dose in the SFC group to the FP group gave a ratio of 0.76 (0.66 - 0.89, p <0.001) at week 2 and 0.81 (0.71 - 0.94, p = 0.006) at week 4. Similarly, significant results in favour of SFC for oscillometry measurements of resistance and reactance were observed. FEV1 was also significantly superior at week 2 in the SFC group (mean difference 0.16L, 95% CI; 0.03 - 0.28, p = 0.015), but not at week 4 (mean difference 0.17L, 95% CI -0.01 - 0.34, p = 0.060).Conclusion: SFC is superior to FP in reducing airway resistance in mild asthmatics with near normal FEV1 values. This study provides evidence that changes in pulmonary function in patients with mild asthma are detected more sensitively by plethysmography compared to spirometry. Trial registration number: NCT00370591. © 2007 Houghton et al; licensee BioMed Central Ltd.