Three hundred and fifty-three asthmatic patients who remained symptomatic despite treatment with budesonide 800-1200 microg day(-1) (or equivalent) were randomized to a new combination therapy comprising salmeterol 50 microg and fluticasone propionate 250 microg (Seretide, Advair, Viani 50/250 microg) twice daily or budesonide 800 microg twice daily for 24 weeks. Patients kept daily records of their morning and evening peak expiratory flow (PEF), daytime and night-time symptom scores and daytime and night-time use of rescue salbutamol. Mean morning PEF increased by 451 min(-1) (baseline 361 l min(-1)) in the salmeterol/fluticasone propionate combination (SFC) group and by 19 l min(-1) (baseline 358 l min(-1)) in the budesonide group over the 24 weeks. The adjusted mean morning PEF over weeks 1 to 24 was significantly greater in the SFC group, despite the > three-fold lower corticosteroid dose (406 vs. 380 l min(-1); P < 0.001). A significantly greater improvement in evening PEF was also seen in the SFC group (adjusted mean 416 vs. 398 l min(-1); P<0.001). SFC also provided significantly better control of daytime symptoms and a significantly greater reduction in the requirement for rescue salbutamol compared with budesonide. These results demonstrate that SFC 50/250 microg twice daily is superior to budesonide 800 microg twice daily in the management of patients with moderate to severe asthma who are symptomatic on their existing dose of corticosteroid.