Clobetasone butyrate 0.05% (Eumovate), a halogenated topical steroid, was compared with hydrocortison butyrate 0.1% (Locoid) which does not contain any halogen atoms. In the treatment of eczema there was not difference between the preparations, but in that of psoriasis the halogen-containing steriod was significantly more effective. Under normal circumstances neither preparation had any detectable effect on adrenal function, but with large doses under total-body polythene occlusion, circulating cortisol levels were reduced less by the halogenated than by the non-halogenated preparation. Corticosteroids which contain a halogen atom are often considered to cause more adverse effects than the non-halogenated preparations with similar clinical efficacy. This study shows that this cannot be assumed for their ability to suppress cortisol levels.