Hydrofluoroalkane-134a (HFA-134a) is a new chlorofluorocarbon (CFC)-free propellant for use in metered dose inhalers. It provides a more environmentally friendly alternative to CFC propellants because it does not contain chlorine which is responsible for ozone depletion by CFCs. Beclomethasone dipropionate (BDP) is widely used for inhalation asthma therapy and is most commonly delivered by a CFC propellant system. The present study evaluated the acute safety of BDP formulated with the new propellant (HFA-134a BDP) compared with BDP in a CFC-11/12 formulation by measuring the acute bronchial response in asthmatic patients. The study was conducted as a randomized, single-blind, placebo-controlled, four-period cross-over trial. Asthmatic patients received eight inhalations of four treatment regimens (HFA-134a BDP, 1600 mg total dose; CFC-11/12 BDP, 2000 mg total dose; HFA-134a placebo and CFC-11/12 placebo) in random order over four study days. Forced expired volume in 1 s (FEV1) was measured before and 2, 10, 20, 40 and 60 min after inhalation of the study treatments. The number of coughs was counted from the start of the first inhalation to 60 s after the last inhalation. There were no statistically significant differences between the treatment groups for changes in FEV1, for the number of coughs or for the occurrence or severity of bronchoconstriction. In asthmatic patients withholding bronchodilators, the new HFA-134a BDP propellant system proved as safe and was as well tolerated as the current CFC-11/12 BDP system. The two propellant systems without active drug were also equally well tolerated.