A method for estimating the regional lung dosages of a nebulized suspension is presented and applied to Pulmicort(R) (budesonide) suspension (4 ml, 0.5 mg/ml) nebulized with three Pari LC+ nebulizers driven by a Pulmo-Aide compressor. The methodology combines experimental measurements of the nebulized aerosol with a mathematical lung deposition model. By adding methylene blue as a tracer for the water, cascade impaction with UV spectrophotometry is used to characterize the distribution of both budesonide and water in the inhaled droplets. Tidal breathing is simulated experimentally using a breath simulator to estimate the amount of inhaled drug. A valve system allows cascade impaction to occur at a constant flow rate of 28.3 l/min. while inhalation at 18 l/min. occurs through the nebulizer. Lung dosages (as % of inhaled dose) obtained with the methodology are in good agreement with values observed in vivo by previous researchers using pharmacokinetic methods with the LC+ nebulizer and the present budesonide formulation. Budesonide is found to be preferentially contained in the larger droplets, and calculated regional lung dosages show that an assumption of homogeneous distribution of the budesonide in the inhaled droplets is incorrect.