A multistage liquid impinger was used to collect the nebulised cloud from three separate nebulisers. The output of sodium cromoglycate collected was determined by a spectrophotometric assay. Estimating drug output purely from weight loss during nebulisation resulted in a considerable overestimate compared with direct assay of the drug output from the nebulised cloud. During nebulisation, weight loss from the nebuliser occurs in the form of particle formation and also by evaporation. By only weighing the nebuliser chamber before and after nebulisation, weight loss due to evaporation is not taken into account and this is the cause of the overestimation of drug output by this method.