There are significant differences in the pharmacokinetic properties of inhaled corticosteroids currently available for use in treatment of asthma and this can result in differences in pharmacodynamic activity. All currently used inhaled corticosteroids are rapidly cleared from the body, but show varying levels of oral bioavailability, with fluticasone propionate having the lowest. Following inhalation, there is also considerable variability in the rate of absorption from the lung, and pulmonary residence times are greatest for fluticasone propionate and triamcinolone acetonide, and shortest for budesonide and flunisolide. Cortisol suppression is frequently used as a surrogate marker of systemic corticosteroid activity. Cortisol release displays a circadian rhythm, which can be mathematically modelled and the effects of exogenous corticosteroids on cortisol suppression established. However, when interpreting the effects of inhaled corticosteroids on cumulative cortisol suppression, it is important to take into consideration the pharmacokinetic properties of each particular drug, together with the study design and the time of administration.