Studies involving the direct measurement of clinical response to inhaled asthma drugs, especially inhaled corticosteroids, may be very difficult to conduct. However, the deposition of drug in the lungs may be considered as a measure of local bioavailability, and may be quantified by radionuclide imaging techniques, or for some drugs by pharmacokinetic methods. This paper reviews evidence for considering lung deposition data as a surrogate for the clinical response to inhaled asthma drugs, based mainly upon a series of case histories. The appropriate use of lung deposition data in regulatory packages, especially to document the equivalence or comparability of two products, offers the possibility of significant time saving in the drug development process, and hence a faster drug development programme for inhaled asthma products.