Background and objective: The effects of adding a second inhaled corticosteroid with a different particle size, compared with using an increased dose of a single inhaled corticosteroid, were assessed in patients with persistent asthma. Methods: This was an open-label study of Japanese asthma patients over 20 years of age. After a 1-month run-in period, 36 patients with inadequate control while using salmeterol/fluticasone propionate 50/250 mu g (SFC50/250) bd, were randomized to receive SFC50/500 bd or SFC50/250 plus mometasone 100 mg bd (SFC50/250/MF100) for 2 months. Results: Both treatments resulted in improvements in morning and evening PEF. There were no significant changes in FEV1, maximum mid-expiratory flow, maximum expiratory flow rate at 50%, maximum expiratory flow rate at 25% or exhaled NO (FENO) in the SFC50/500 group. On the other hand, there were significant improvements in FEV1% (+ 12.2%, P = 0.0142), % maximum mid-expiratory flow (+ 28.9%, P = 0.0181), % MEF50 (+ 32.4%, P = 0.0206) and % MEF25 (+ 30.3%, P = 0.0113) in the SFC50/250/MF100 group. The changes in FENO (-23.2% (P = 0.0157) in the SFC50/250/MF100 group and -14.5% (not significant) in the SFC50/500 group) did not differ significantly between the groups. Conclusions: In patients with severe persistent asthma, addition of low-dose mometasone to SFC50/250 improved spirometric parameters, FENO and PEF, while an increase in dose from SFC50/250 to SFC50/500 only improved PEF.