The present study demonstrates the equivalent efficacy for BDP 500 microg bid given via MDI with the new HFA-134a propellant (Chiesi Farmaceutici S.p.A., Parma) compared to a conventional CFC propellant (Becotide, Allen & Hanburys, UK). One hundred and sixteen adult patients with stable mild to moderate asthma (FEV1 > or = 60% of predicted normal) entered a 2-week run-in period where they maintained their own inhaled corticosteroids and were then assigned to a 12-week treatment with the test drug in a randomized, multicentre, double-blind, double-dummy, parallel-group design. Ninety-one patients completed the study period. Morning and evening peak expiratory flow rate (PEFR), use of rescue salbutamol, number of daytime and nighttime asthma attacks, number of nighttime awakenings, and clinical symptoms were recorded daily by patients on a diary card. Pulmonary function tests (FEV1, FVC, PEFR, MEF50 and FEF25) were completed at study entry, at the start of treatment and every 2 weeks thereafter. Morning (08.00-10.00 AM) serum cortisol was measured at the start and at the end of treatment. Adverse events were collected for the total study period. Equivalence between groups was demonstrated for the primary end-point morning PEFR, as well as for evening PEFR and FEV1 (the 95% CI of the treatments' difference was within the 5% of the LSM of BDP CFC). The other secondary pulmonary function tests measured at the clinic visit showed a satisfactory asthma control, albeit without statistically significant differences between groups. Decreases in the use of rescue salbutamol and in clinical symptoms were also reported in both groups, with no differences between them. Adverse events were reported in 81.4% of patients in the BDP HFA group and in 82.5% in the CFC group. There were 73 and 59 adverse drug reactions in the two groups, respectively; the difference was mainly due to differences in taste. No drug-related serious adverse events were reported in either group. No difference was seen for morning serum cortisol between baseline and end of treatment, or between groups. In conclusion, the BDP-HFA 134a formulation proved to be statistically equivalent to the standard BDP CFC product over 12 weeks in adult patients with mild to moderate asthma.