The safety of long-term inhaled corticosteroid therapy at commonly prescribed doses is an issue of growing concern to physicians and international regulatory bodies. This is so because long-term use of these drugs has become the mainstay of chronic asthma management and their introduction now is widely recommended in official treatment guidelines at the 'mild persistent' stage of asthma, where regular daily therapy is first begun. In addition to more frequent use of inhaled corticosteroids, there is a further trend to use higher doses of existing inhaler therapies and to use the newer and more potent compounds that have recently become available. At the same time as these developments have been taking place, there has not been a concurrent move to a more rigorous examination of the safety profile of these inhaled corticosteroid treatments - especially to assess their effects on the hypothalamic-pituitary-adrenal (HPA) axis. Most safety data with respect to HPA axis effects have been derived from testing methods that are limited in their ability to detect HPA system impairment and, more seriously, that can give the impression of functional integrity in the HPA axis when there may be moderate (or even greater) impairment. In this first part of a two-part review of the HPA axis effects of inhaled corticosteroids and of how these effects should be assessed, we examine the currently used and the currently available testing methodologies and also review the present state of knowledge concerning the structure and function of the HPA axis and the effects of its suppression. It is clear that there are state-of-the-art tests to assess in a discriminating manner the safety profile of inhaled corticosteroids. These tests have been insufficiently employed, including during the drug development process, yet they are readily available, relatively inexpensive and can detect adrenal suppression before the appearance of clinical effects. In part 2 of this review we examine what can be learned about the effects of inhaled corticosteroid therapy on the HPA axis from the limited amount of reliable published information from clinical and pharmacological studies describing their use and safety.