The particle size distribution of an aerosol generated from an aqueous system is difficult to analyse because of the shrinkage of the droplets due to solvent evaporation. These problems are very important for the characterization of medical nebulizers, since most of the drugs delivered via inhalation are water soluble. In situ methods for droplet size analysis, such as laser diffraction, phase Doppler anemometry and light scattering, do not determine either the initial or the equilibrium size distribution. With the residual technique, which means evaporating the droplets and measuring the size and concentration of the residuals, the instability of the aqueous droplets plays no role and the necessary radioactive labelling of the sprayed material allows a direct determination of the mass flow rate at the mouthpiece of the nebulizer. Tn this way it is possible to discriminate between the delivered drug solution and the water necessary to humidify the incoming air. The output of nebulizers of different designs is given for various operating conditions, filling volumes and solution concentrations. The measured droplet size distribution of a nebulizer is found to be fixed mainly by its internal impaction system.