The Diskus/Accuhaler inhaler (D/A) is a new multidose powder inhaler, designed to deliver all asthma drugs. This study was carried out to establish clinical equivalence between FP 250 microg twice daily (b.i.d.) via the D/A and FP 250 microg b.i.d. via the current powder inhaler, the Diskhaler (DH). Also, device handling and patient preference for both devices were determined. This was a multicenter, randomized, double-blind, double-dummy, parallel-group study. Adult asthmatics (364, aged 18-79) requiring inhaled corticosteroids in a daily dosage of 400 microg up to and including 1000 microg and demonstrating a mean morning peak expiratory flow rate (PEFR) calculated from the last 7 days of the run-in of less than 85% of the response after salbutamol, a baseline forced expiratory volume in 1 sec (FEV1) between 50 and 90% of their predicted normal value, and an ability to correctly use both devices, were randomized to a 12-week treatment period. No statistically significant differences between the two devices were seen for mean morning PEFR (p = 0.76), mean evening PEFR (p = 0.88), median daytime and nighttime symptom score (p = 0.57 and p = 0.47), median percentage of rescue-free days and nights (p = 0.43 and p = 0.24), and clinic visit lung function. No differences in the safety profile of FP were seen. Patients found the D/A easier to use and easier with respect to assessing the number of doses remaining (both p < 0.001). Sixty-five percent of the patients expressed an overall preference for the D/A over the DH (p < 0.001). The results show that FP 250 microg b.i.d. via the D/A is clinically equivalent to delivery via the DH. Both treatments proved to be equally safe and were well tolerated. The D/A was easier to use and patients preferred the D/A over the DH.