Ketoconazole, an oral antimycotic agent, is known to have a potent inhibitory effect on adrenal steroid production. It was given at a dose of 1200 mg/day to a 52-year-old female patient with a virilizing adrenocortical carcinoma in order to achieve better metabolic control pre-operatively. Together with a rapid normalisation of hypertension and hyperglycemia, a dramatic fall was noticed in serum and urinary adrenal steroids after a few days. Levels of total testosterone (20 nmol/l), androstenedione (>35 nmol/l) and DHEA-sulphate (>28 nmol/l) fell to normal levels in 6 days. By contrast, levels of 17-OH\x=req-\ progesterone (30 nmol/l) and progesterone (2.45 nmol/l) increased slightly, indicating inhibition af adrenal 17, 20\ x=req-\ lyase. Cortisol (620 nmol/l at 08.00 h) fell to very low levels (50 nmol/l) on day 6 of the trial. We conclude that ketoconazole is very effective in suppression of adrenal tumoural steroidogenesis and merits consideration in pre-operative use. We warn against dangerous hypoadrenalism which seems to occur earlier in tumoural than in normal adrenal metabolism.