This study compares percutaneous absorption of hydrocortisone after short and long-term administration. The experimental design was to first apply 14C-hydrocortisone. This was followed by long-term administration of nonradioactive hydrocortisone. Then 14C-hydrocortisone was applied again when urinary excretion of radioactivity from the first application reached minimum detectable levels. Short- (first 14C-hydrocortisone application) and long-term (second 14C-hydrocortisone application) penetrations were compared. The level of absorption of hydrocortisone significantly increased during long-term administration, whether applied in an acetone vehicle or in a 0.9% emulsion ointment base (Eucerin). A placebo study in which only an acetone vehicle was applied for a long period followed by 14C-hydrocortisone application showed no enhanced penetration. It is suggested that long-term application of hydrocortisone alters the penetration barrier, resulting in enhanced penetration. The relevance of this long-term corticosteroid use in man is possibly of great importance.