Background: Corticosteroids may cause immediate or delayed hypersensitivity. In 1989, based on structural and clinical characteristics, we put forward a classification of corticosteroids into four crossreacting groups, namely group A, B, C, and D, the latter later subdivided into two subgroups, i.e. D1 and D2. The constituents on the D-ring of the corticosteroid-molecule are considered to have a central role for binding to skin proteins and for cross-reactions patterns; however, halogenation of the molecules is also interfering.