The use of breathing simulators for the in vitro determination of the inhaled mass of drug from nebulizers has become widely accepted. Their use is, however, based on the assumption that there is a correlation between the in vitro and in vivo inhaled mass of drug. The aim of the study was therefore to investigate whether a new breathing simulator--the MIMIC Breathing Emulator (Medic-Aid Limited, Bognor Regis, UK)--could accurately emulate the in vivo inhaled mass of budesonide suspension for nebulization. Eight adult healthy subjects were included. Each subject inhaled for 2 min from a Spira Module 1 jet nebulizer (Respiratory Care Center, Hämeenlinna, Finland), charged with 1.0 mg of budesonide suspension for nebulization (0.5 mg mL-1, 2 mL suspension, AstraZeneca, Sweden) and supplied with an inhaled mass filter between the nebulizer and the subject. The breathing patterns were recorded during the nebulization and simulated in vitro at two different experimental sites (simulations A and B) with the breathing simulator. With the patients breathing through the filters (in vivo test), inhaled mass of budesonide averaged 103.6 micrograms. The two in vitro experiments using the simulator revealed similar results with in vitro simulation A equal to 101.0 micrograms and simulation B 99.1 micrograms. There were no statistically significant differences between the in vivo results and those of in vitro simulation A. Results were significantly different for simulation B (p = 0.032) although the difference was less than 4.5%. These data indicate that the breathing simulator can be used to accurately simulate sine waveforms, human breathing patterns, and the in vitro and in vivo inhaled mass of budesonide suspension for nebulization.