The aim of this study was to compare the efficacy safety and cost of Seretide (salmeterol/fluticasone propionate (Salm/FP), 50/250 microg bd) via Diskus with formoterol (Form; 12 microg bd) and budesonide (Bud; 800 microg bd) given concurrently (Form+Bud) via Turbuhaler in patients with moderate-to-severe asthma who were uncontrolled on existing corticosteroid therapy. The study used a randomised, double-blind, double-dummy, parallel-group design, consisting of a 2-week run-in period on current corticosteroid therapy (1000-1600 microg/day of BDP or equivalent) and a 12-week treatment period. Symptomatic patients (n = 428) with FEV1 of 50-85% predicted and increased symptom scores or reliever use during run-in were randomly allocated to receive either Salm/FP (50/250 microg bd) via a single Diskus inhaleror Form+Bud (12+800 microg bd) via separate Turbuhalers. Clinic, diary card and asthma-related health-care resource utilisation data were collected. Improvement in mean morning peak expiratory flow (PEFam was similar in the Salm/FP and Form+Bud groups. Both PEFam and mean evening PEF (PEFpm) increased by a clinically significant amount (>20 L/min) from baseline in both treatment groups. The mean rate of exacerbations (mild, moderate or severe) was significantly lower in the Salm/FP group (0.472) compared with the Form+Bud group (0.735) (ratio = 0.64; P < 0.001), despite the three-fold lower microgram inhaled corticosteroid dose in the Salm/FP group. Patients in the Salm/FP group also experienced significantly fewer nocturnal symptoms, with a higher median percentage of symptom-free nights (P = 0.04), nights with a symptom score <2 (P = 0.03), and nights with no awakenings (P = 0.02). Total asthma-related health-care costs were significantly lower in the Salm/FP group than the Form+Bud group (P<0.05). Both treatments were well tolerated, with a similar low incidence of adverse events. This study showed that in symptomatic patients with moderate-to-severe asthma, Salm/FP (50/250 microg bd), administered in a single convenient device (Diskus), was at least as effective as an approximately three-fold higher microgram corticosteroid dose of Bud (800 microg bd) given concurrently with Form (12 microg bd) in terms of improvement in PEFam, and superior at reducing exacerbations and nights with symptoms or night-time awakenings. Salm/FP was also the less costly treatment due primarily to lower hospitalisation and drug costs.