It has been proposed that corticosteroid cross-reactions occur more frequently within structurally well-defined groups. To test this hypothesis we have compared the patch-test reactions to other corticosteroids in 96 patients allergic to hydrocortisone. We found that our data did not agree with the previously proposed classification. The presence of a substitution at the C6 or C9 position was the most important factor in determining whether a patient would be allergic to another corticosteroid. This information should facilitate the choice of an alternative corticosteroid in patients allergic to hydrocortisone, if facilities for patch testing to other corticosteroids are not available.