Summary: Inhalation therapy with nebulized steroids (e.g. budesonide) is possible only if inhaled aerosol droplets contain micronized solid particles of the active ingredient. Since both aerosol droplets and budesonide particles have comparable sizes (micrometer-range), it is essential to prove that drug particles are indeed present in the aerosol released from the nebulizer. This work presents data of the study focused on the particle size of budesonide in several drug products. Particle size determination was done in liquids sampled directly from nebulizers (mesh and pneumatic type) during their normal operation. It was found that at such conditions the majority of budesonide particles in all tested drugs (Benodil, Budixon Neb, Nebbud, Pulmicort) are smaller than 3–4 mm. Since the average diameter of droplets (D50, MMAD) in the aerosol released from studied nebulizers is in the range of 5–6 mm, the results confirm that in all tested cases the aerosol should contain a similar amount of steroid particles. Small differences in budesonide particle and aerosol droplet size distributions which were found in various nebulizer-drug combinations are unimportant for the predicted penetration of aerosolized drugs to the respiratory system.