To evaluate the effect of topical hydrocortisone therapy on cortisol secretion, the plasma cortisol response to a 2 h adrenocorticotropic hormone (ACTH) test was determined in 17 children with dermatitis. Percutaneous absorption of hydrocortisone was measured in the acute phase of dermatitis by a 4 h absorption test. Two-hour plasma cortisol in the ACTH test correlated inversely with the increment of plasma cortisol in the absorption test. The duration of topical hydrocortisone therapy had no significant influence on the adrenocortical response to ACTH. A moderately or severely subnormal 2 h plasma cortisol level was observed in three infants after 3-5 months' topical therapy with hydrocortisone.