The study objective was to compare the effect of budesonide administered as a nebulized suspension as compared to a spray with a spacer in adult asthmatics. In a double-blind, double-dummy crossover study, 26 adult patients with moderately severe unstable asthma were randomized to three 4-week treatment periods with budesonide 0.8 mg b.i.d. administered by a pressurized metered-dose inhaler (pMDI) with spacer (Nebuhaler) and budesonide 1 mg and 4 mg b.i.d. administered by a Pari Inhalier Boy jet nebulizer. The nebulizer was activated only during inspiration. The total mass output was similar from the two devices but their fraction of small particles differed by a factor of 2 in favour of pMDI. Effect was evaluated from daily home measurements of peak expiratory flow (PEF), need of beta 2-agonist and symptom scores. Plasma cortisol and budesonide levels were measured in a subgroup of 10 patients. A consistent trend showed the nebulizer treatment to be at least as efficient as the pMDI plus spacer treatment. In actual fact, the apparent order of effect was: 4 mg nebulized suspension treatment > or = 1 mg nebulized suspension treatment > or = 0.8 mg pMDI with spacer treatment. Plasma budesonide and plasma cortisol also exhibited dose-related levels independent of device. The adverse effects reported appeared to be related to the dose rather than delivery device. Accordingly, the effect was related to total mass output, rather than to the small particle fraction of the budesonide aerosol. These results attest to the efficiency of jet-nebulized budesonide suspension, and indicate nebulized budesonide to be equipotent to standard budesonide therapy delivered by pMDI with Nebuhaler, provided nebulization is synchronized with inspiration and no loss of aerosol occurs during expiration.