A virilizing adrenocortical adenoma was demonstrated in a yound female. Urinary 17-ketosteroid excretion and subfractions, plasma testosterone level and urinary 17-ketogenic steroid excretion were markedly increased. Dehydroepiandrosterone was the main constituent of the androgen excess. Otherwise the adrenocortical function was found to be normal as evaluated from measurements of cortisol, corticosterone and their metabolites under basal conditions and during dynamic tests. The androgen excess showed an unexpected response to trophic hormones. Human chorionic gonadotrophin stimulation resulted in a pronounced increase in androgen production, whereas no gonadotrophin-dependency could be demonstrated by means of the oestrogen suppression test. Similarly, no corticotrophin-dependency could be demonstrated by corticotrophin stimulation and suppression tests. Removal of the tumour resulted in normalization of the androgen production and no abnormal response upon human chorionic gonadotrophin administration could now be found. The inappropriate response of tumours to trophic hormones is discussed. It is concluded that the reliability of stimulation and suppression tests in determining the site of excessive androgen production should be accepted with reservation.