Percutaneous absorption of hydrocortisone was studied in 18 children (aged from 6 weeks to 14 1/2 years) with atopic or seborrhoeic dermatitis, by measuring their serum cortisol before and after application of 1% hydrocortisone cream. Endogenous secretion of cortisol was suppressed with dexamethasone. A 24 h absorption test was performed on nine children. In six, percutaneous absorption was detected. The highest serum cortisol level was reached within the first 6 h. A 4 h absorption test was developed on the basis of the 24 h test. This short absorption test was performed on nine children, and in eight of them absorption of hydrocortisone was detected. The rise of serum cortisol ranged from 98 to 2669 nmol/l. The 2 h ACTH test was performed to evaluate the effect of previous treatment with topical glucocorticoids. Suppressed adrenocortical function was found in five of 13 children, and was associated significantly with high post-application serum cortisol levels. This occurred more often in infants with a severe skin disorder than in older children or in those with mild or moderate skin disease.