Flunisolide is a synthetic corticosteroid approved for the treatment of persistent asthma and delivered by means of a metered-dose inhaler (MDI). A new formulation of flunisolide, using hydrofluoroalkane (HFA) as a propellant, has been developed to comply with the mandated worldwide phase-out of ozone-depleting chlorofluorocarbon (CFC) propellants. Aerosol particle size in the new flunisolide HFA solution is smaller than the flunisolide CFC suspension (1.2 vs 3.8 microm mass median aerodynamic diameter). Aerosol particle size is a key element in determining lung deposition and the regional distribution of inhaled medication within the lung. In addition, the flunisolide HFA MDI has been redesigned to include a built-in spacer. These features have improved distal lung deposition. Flunisolide HFA, at one-third the dosage (170 and 340 microg twice daily), had similar efficacy to flunisolide CFC (500 and 1000 microg twice daily) and significantly greater efficacy than placebo in a randomized, double-blind, placebo-controlled, 12-week study in patients with mild to moderate asthma. Flunisolide HFA was well tolerated in all trials. A long-term study found no suppression of adrenal function and minimal systemic effects were observed both in adults and children.